John Theodore Buchholz

Developmental Selection in Vascular Plants

Embryonic selection and gametic selection represent the important forms of developmental selection found among pteridophytes. Among gymnosperms the gametophytes plus the embryo usually take part in this competition, and among angiosperms it is typically a competition between male gametophytes represented by the pollen tubes. The important reproductive developmental process is merely shifted to an earlier stage of the life cycle in passing from ferns to angiosperms, where vegetative and other less important forms of developmental selection are also found.

Origin of Cleavage Polyembryony in Conifers

This discussion shows that cleavage polyembryony is a feature of the embryogeny which must have had its origin before the developmental selection within an ovule had become adjusted, under conditions when only one egg at a time was fertilized. It is possible that this occurred during the stage of transition from independent microgametophytes to pollen tubes, or it may have appeared after this time. In any event, it must have occurred before the present adjustment was perfected wherein pollen tubes take part in the process of developmental selection, and cleavage polyembryony is therefore a palingenetic character. The destructive competition which may be recognized in conifer embryos favors the development of stiff, extremely long suspensors, and also the reversion from cleavage to simple polyembryony. Morphological evidence is presented which supports the explanation that most of the living conifers whose embryogenies are known, perhaps all of them, have passed through a stage of cleavage polyembryony. The latter is therefore a primitive feature so far as conifers are concerned, although some of them have retained cleavage polyembryony as a specialization.

Generic and Subgeneric Distribution of the Coniferales

Cycadophyten aus den Lunzer Schichten: Makrosporophylle. Denkschr. Akad. Wiss. Wien, Math.-Naturwiss. K1. 97. I9I9. 6. MCCEELLAND, J. Report of the Geological Survey of India for the season I848-49 Calcutta. I 850. 70 OLDHAM, T. and MORRIS, J. Fossil flora of the Rajmahal series in the Rajmahal Hills. Palaeontologia Indica. The fossil flora of the Gondwana system I: I-52. I863. 8. RAO, A. R. The structure and affinities of TaenioSteris pat?lata McCl. Proc. Nat. Acad. Sc i . (Irl dia) I 3: 3 3 3-3 5 5 I 943 9. SAHNI, B. The anatomy of Nephrolepis vollfbilis J. Sm., with remarks on the biology and morphology of the genus. New Phytol. I4:25I-274.

Volumetric Studies of Seeds, Endosperms, and Embryos in Pinus ponderosa During Embryonic Differentiation

1. Data were obtained on the seeds, endosperms, and embryos from cones of previously described trees of Pinus ponderosa. Volumetric measurements were made, cotyledons were counted for embryos from different sizes of seeds, and the rate of growth of the embryos was computed. 2. For a period of about 5 weeks there is little embryonic differentiation, the embryos within each seed being engaged in a competitive struggle for survival-a period of embryonic selection. 3. After the dominant embryo has been established, it spurts ahead in rapid growth and differentiates all its organs in a period of 10-12 days. Seed ripening occurs in a few weeks with little change in embryo size. 4. A great diversity of stages is found among embryos collected on a given date. This makes possible a computation of the presumable rate of growth between successive collections of seeds from the same tree. 5. The average rate of growth is about 1/2 stage per day during embryo differentiation. It is much slower in the smallest seeds. 6. At the time of shoot-tip differentiation and of cotyledon initiation, the volume of the embryo is closely related to the volumes of endosperm and of seed. 7. There exists a correlation among seed size, embryo size, endosperm size, and number of cotyledons in the embryo.

The Suspensor of Sciadopitys

1. The early embryo of Sciadopitys shows two or three regions or tiers of cells which may develop into later stages. The cells of one of these regions elongate to form the prosuspensor; those of another represent embryo initials situated below the prosuspensor; and those of the third, which may be present, are rosette cells above the prosuspensor. A cap cell consisting of a terminal aborted embryo initial may be present, and other embryonic cells anywhere in the embryo system may become aborted. 2. As the prosuspensor cells become fully elongated, the embryo initials give rise to embryos which are pushed out on single-celled, primary suspensors. The early embryos pass through a stage in which an apical cell is recognizable. Rosette cells, when present, are embryo initials. These may give rise to embryos which are usually aborted in early stages. 3. Embryonal tubes are formed which reinforce the primary suspensor, thus giving rise to a multicellular secondary suspensor. 4. The potential output per archegonium is 12-28 embryos, which may develop independently for a time. If three eggs are fertilized, an embryo complex may be formed with several times this number of separate embryos. 5. Embryos may undergo budding and twinning in later stages; however, the seed usually contains only a single embryo which is fully matured with cotyledons. The number of cotyledons is usually two but occasionally three. 6. The stem tip is not organized until long after the cotyledons are formed. 7. The root cap is relatively short, and merges very gradually into the secondary suspensor. 8. The embryo of Sciadopitys is a type from which the embryos of the Cupressineae may have been derived; it also represents a type from which the embryos of the Taxodineae may have been derived. It has probably had a common origin with the embryo type represented by Cephalotaxus and certain podocarps, and appears to be more distantly related to the Abietineae.

Pollen Tube Growth and Control of Gametophytic Selection in Cocklebur, a 25-Chromosome Datura

1. Pollen tube distributions were plotted from 12-, 16-, and 18-hour tests, using the pollen of the primary (2n + 1) Cocklebur germinated and grown on the pistils of 2n plants. These tests were compared with similar tests of pollen tube growth of normal plants in line 1a. 2. Further evidence is given from the results of ordinary male back-crosses of Wedge to four lines, indicating that the latter is a secondary (2n + 1) type related to Cocklebur. Pollen tube distributions obtained from the pollen of Wedge are also plotted. 3. The pollen tube distribution curves indicate that Cocklebur may be transmitted through the pollen under conditions in which the processes of gametophytic selection are controlled experimentally. Likewise, the distribution curves indicate that Wedge may not be transmitted through the pollen in line 1A. 4. The correctness of the interpretations gained from pollen tube distribution curves was tested genetically in the case of Cocklebur, by combining three other experimental methods: the effects of restricted pollinations; the effects of excision of styles; and the separation of the seeds in the lower half of seed capsules from those in the upper half. Results from all experimental methods confirmed the interpretations gained from a study of pollen tube distribution curves. 5. The various methods which have been used by investigators at different times in a study of the effects of gametophytic selection are briefly discussed and compared. 6. The pollen tube distribution curves obtained from suitable tests enable us to recognize processes of gametophytic selection, and have the further advantage of enabling us to distinguish between at least five different conditions, all of which may lead to the complete or partial elimination of some of the classes of male gametophytes, resulting in modified ratios in the progeny. 7. The processes of gametophytic selection involved in the pollen transmission of Cocklebur may be controlled through a wide range. A threefold increase may be obtained by restricted pollinations, and a further increase, up to 65 per cent Cocklebur plants, may be obtained in a progeny by selecting the seeds to be planted from the lower parts of seed capsules, while complete non-transmission of Cocklebur may be obtained by properly timed excisions of styles.

Embryogeny of the Podocarpaceae

Introduction The genus Podocarpus is divided, according to PILGER (20), into two subgenera -Stachycarpus and Protopodocarpus. The latter subgenus is further subdivided into several sections: Dacrycarpus, Microcarpus, Nageia, and Eupodocarpus. Several years ago the writer (4) contributed a series of stages in the embryogeny of several species of Stachycarpus. The embryogeny of Dacrydium cupressinum had been described earlier (3). The present paper is concerned chiefly with the embryogeny of Dacrycarpus, Nageia, and Eupodocarpus. The embryogeny of additional representatives of the subgenus Stachycar pus and of Phyllocladus is described more fully than formerly, with special reference to details concerning the binucleate embryonic and apical cells. This paper records practically all the properly preserved embryological material which it has thus far been possible to obtain from conservatories in this country, from correspondents in the Southern Hemisphere, or from collections made by American botanists while abroad. I am indebted to Mr. H. W. Lawton of Wellington, New Zealand, for collections of Podocarpus totarra nd several other species; to Dr. E. W. Sinnott for P. dacrydioides; and to Dr. M. S. Markle for specimens of P. urbanii obtained on Blue Mountain, Jamaica. The material of P. macrophyllus maki (P. chinensis) was collected by the writer from the conservatory of the New York Botanical Garden. While I have observed many species of the Podocarpaceae on estates and in parks and public gardens in California, very few of the species grown in this country, aside from P. macrophyllus, were found to produce seeds. Some in California produce pollen cones and ovules, but unfortunately many of the plantings of rare species are so scattered as isolated specimens that

Polyembryony Among Abietineae

1. Although all species of Pinus have shown a complete separation of the 4 primary embryos, this feature of cleavage polyembryony is not characteristic of all Abietineae. 2. The cleavages which separate the 8 embryos from each other are the free nuclear divisions of the proembryo. In forms without cleavage polyembryony (Picea, and as far as we know concerning other forms), cell divisions homologous with those in Pinus occur in the proembryo. 3. The embryos of the Abietineae may be arranged in an intergrading series, with Pinus at one end and Pseudotsuga at the other, on the basis of the occurrence of cleavage polyembryony, rosette embryos, and the apical cell. The rosette embryos and their vestiges, the rosette cells, are gradually eliminated as we pass from Pinus to Pseudotsuga. 4. Cleavage polyembryony, rosette embryos, and the apical cell mark a primitive type of embryo development. 5. The embryo development of this group shows how the apical cell was lost in the evolution of the Abietineae. 6. On the basis of embryogeny Pseudotsuga is unique and is entitled to rank as a separate genus.

Determinate Cleavage Polyembryony, with Special Reference to Dacrydium

1. A distinction may be made between the two types of cleavage polyembryony: indeterminate and determinate. 2. The embryogenies of Pinus, Cedrus, Tsuga, Cryptomeria, Biota, and Chamaecyparis illustrate the indeterminate type of cleavage polyembryony. 3. The embryogeny of Dacrydium cupressinum is described in some detail and used to illustrate an embryogeny with determinate cleavage polyembryony. 4. A prosuspensor but no primary suspensors are found in Dacrydium. 5. No traces of rosette cells were observed in Dacrydium. 6. The early embryo system of Dacrydium may be recognized as being compound or composite through binucleate embryo initial cells borne on the prosuspensor. 7. The individual embryos of Dacrydium were found to develop by apical cell growth, beginning with the formation of walls in the initial cells and continuing to a stage of more than 75-80 cells; but an apical cell was not found in embryos of three or four times this number of cells. 8. The embryogenies of conifers may be separated into at least four levels with reference to indeterminate and determinate cleavage polyembryony, and simple polyembryony with or without vestigial cleavage polyembryony. 9. These new distinctions in types of cleavage polyembryony or simple polyembryony facilitate the comparison of embryogenies.

The Suspensor of Cryptomeria japonica

The outstanding features found in the embryogeny of Cryptomeria are cleavage polyembryony, the absence of typical primary suspensors, and the apical cell method of growth in the early embryo.