A.R.A. Noel

Gall formation in Erythrina latissima

Summary The Chalcid galls that develop on the leaves of Erythrina latissima are morphologically consistent, suggesting that they either have an obligate fauna, or alternatively that one organism, probably a species of Eurytoma , dominates the whole system. Structurally the galls are as complex as Cynipid galls, consisting of four distinct zones. Of these the nutritive and protective zones are the most important as they provide the food for the developing larvae and protection for the pupae when they overwinter in the gall on the ground. It would appear as if the larvae within the galls exert a strong mobilizing effect and creates a strong sink which ensures the translocation of nutrients towards the galls. This is usually manifested in that the leaf greatly decreases in size with increased infestation.

Nicotiana tabacum Callus Studies I. Assessment of Growth and Differentiation

Summary Nicotiana pith expiants containing bundles of internal phloem attain the same plateau of growth in terms of mass and volume after 14 days as do expiants devoid of internal phloem after 21 days. The lag appears to be related to the time required for the dedifferentiation of the quiescent pith parenchyma. Tracheary element differentiation occurred in controls as well as in all treatments with only indoleacetic acid (IAA), 0.3 mgl −1 being significantly effective. Expiants treated only with kinetin (K) developed small numbers of tracheary elements at all concentrations. Sieve elements were not induced in response to media including K but without IAA or with a high level of IAA (3 mgl −1 ) in the absence of K. At low and intermediate levels of IAA phloic tissue differentiated at all levels of sucrose (2, 4 and 6%) with the exception of 0.03 mgl −1 IAA, 1 mgl −1 K and 2% sucrose. There was a lack of consistency in the extent of sieve element differentiation. However, when comparing individual combinations of IAA and K among various sucrose levels, more phloem was initiated at 4 than at 2 or 6%.

Nicotiana tabacum Callus Studies IV. Anatomy and Differentiation of Vascular Tissue

Summary A Nicotiana pith explant behaves like a xylem explant in that, generally, normally-orientated cambium-like zones form in the proliferating marginal tissue, producing tracheary elements centripetally toward the original explant and sieve elements centrifugally toward the callus periphery. Vascular tissue in treatments of 0.3 and 0.1 mgl−1 indoleacetic acid and kinetin, respectively, at all levels of sucrose (2, 4, 6%), was fairly highly organized, being grouped in vascular bundle-like formations similar to those seen in transverse sections of Nicotiana internodes. Isolated nodules of vascular tissue were mostly amphicribal but amphivasal nodules also occurred. Tracheary elements differentiated before sieve elements. It is difficult to assign distinct and exclusive effects on the differentiation of vascular elements to any particular one or combination of substances.

Seed Coat Structure and Germination in Podocarpus henkelii

Summary Imbibition of the Podocarpus seeds is slow and germination, under natural conditions, is delayed. By measuring the imbibition of seeds after removal, successively, of the epicuticular wax, the epidermis and the entire epimatium, it was shown that a coat barrier to water uptake existed. Treatments which increased the rate of imbibition, enhanced germination, from about 70 % after at least 2 months, to 100 % within a few days. The optimum imbibition is 2 days, longer depressing germination, even although uptake may continue for over 6 days. There is no evidence of embryo or oxygen-imposed dormancy. Germination occurs in light or dark, at an optimum temperature of 30 °C. The epimatium slows water loss; nevertheless the seed has little resistance to desiccation and 50 % depletion was recorded. Seeds can be stored successfully however, at 4 °C with high humidity. The wax-covered stomata do not playa significant part in water absorbtion or loss, but peri-stomatal fissures which develop as a result of seed shrinkage, may facilitate water uptake under field conditions.

Nicotiana tabacum Callus Studies II. Variability in Cultures

Summary The original position of an expiant in a Nicotiana stem does not appear to affect growth potential of the cultures in any systematic way; the donor plants themselves are greater sources of variation. Whereas several merits are associated with the use of pith expiants in studying vascular tissue differentiation, callus cultures derived from continuous subculturing are potent sources of variability in this regard. Adequate replication and random allocation of expiants to treatments are imperative if the high degree of variability found in tissue cultures is to be reduced.

Nicotiana tabacum callus studies V. Ontogeny and structure of phloem

Summary Apart from a relative lack of organization phloem histogenesis in Nicotiana pith cultures is broadly similar to that in the intact plant. Phloem tissue in in vitro cultures arises from internal divisions of callus parenchyma — simulating the origin of internal phloem bundles in the intact stem — as well as from mother cells cut off from procambium-like initials — resembling the origin of external phloem in the intact stem. Sieve elements have end walls ranging from transverse to very oblique and bear either simple or compound sieve plates but, although in this way corresponding with primary and secondary type sieve elements, they are not associated respectively with other features of the phloem classed in the intact plant as primary and secondary.

Nicotiana tabacum callus studies. VI. Quantitative hormonal effects on tracheary element differentiation

Summary Evidence is presented that cytokinins and gibberellins act with auxin in promoting xylem differentiation in Nicotiana callus cultures. The interaction between IAA and both kinetin and GA is especially marked for tracheary element induction. IAA was not on the whole as effective at inducing xylogenesis as were combinations of IAA and kinetin, and IAA and GA. It appears that callus tissues undergoing extremely rapid growth are unlikely to undergo correspondingly extensive tracheary element differentiation. However, in IAA: kinetin treatments a positive correlation exists between the extent of callus growth and tracheary element differentiation. It is concluded that tracheary elements induced in vitro are non-functional. High coefficients of variation appear to be a characteristic limitation of tissue culture experimentation.

Nicotiana tabacum Callus Studies III. Size and Shape of Vascular Elements

Summary Tracheary and sieve elements induced in vitro in Nicotiana pith callus are as wide as but about five times shorter than those in the internodes of the plants from which the expiants derive. Shape, especially in the tracheary elements, is extremely irregular. Giant tracheidal cells differentiate in the presence of indoleacetic acid and gibberellin in explants which otherwise have undergone limited amounts of callus development. These tracheids are more regular in shape and are considered to be the direct redifferentiation products of the pith parenchyma cells. Although the sieve elements show much less variation in shape than do the tracheary elements, the slopes of their end walls are more varied than those of sieve elements in the intact plant.

Nicotiana tabacum callus studies: VII. Quantitative hormonal and sucrose effects on sieve element dimension

Summary The effects of auxin, cytokinin and sucrose on sieve element dimensions were investigated in Nicotiana callus cultures. Although significant differences in both mean length and mean diameter were established between various individual treatments, there was no consistent response to increasing the level of any one substance. The treatment that produced the anatomically most advanced vascular tissues and the greatest amount of phloem, namely 0.3 mgl −1 IAA, 0.1 mgl −1 kinetin (K) and 2% sucrose, also resulted in the longest and widest sieve tube members. Raising the sucrose level from 2% to 4% resulted in a significant decrease in length and diameter at all ratios of IAA: K = 3:1 (i.e. 0.03: 0.01, 0.3:0.1 and 3.0:1.0 IAA: K, mgl −1 ).

The Food Reserves and Ultrastructural Relationships of the Gametophyte and Sporophyte Tissues in Podocarpus henkelii Seeds

Summary Tissue of mature seeds of Podocarpus henkelii was fixed in glutaraldehyde and post-fixed in osmium tetroxide for electron microscopy. The major food reserve in both the gametophyte and sporophyte is starch, contained in amyloplasts. Well defined protein bodies without crystalline structures were found in the cotyledons. In the rest of the embryonic tissue, these were very sparse. The inner surfaces of the gametophyte cells in contact with the embryo are transversely ribbed. This greatly increases the area of the plasmolemma, which closely follows the wall contours. In view of their structure and function these cells are considered to be transfer cells, and confirms their existence in gymnosperms.