Seed physiology: From ovule to maturing seed

1. The future of the seed is partly predetermined by events (flower formation, flowering, nutrient flow from mother plant, etc.) preceding fertilization and the formation of the gametophyte.
2. The environmental conditions under which the seed matures affect its final physiological constitution. This faet has mostly been neglected by seed physiologists.
3. It is not known how far the triantic nature of the diaspore (seed coat, pulp, etc., 2n of mother plant, embryon of δ +n of Φ, endosperm 2n of Φ +n of δ) affects seed development and germination.
4. The integuments of the ovules of some species have stomata. It is not known if they are functional in gas exchange or are constitutional non-functioning relics.
5. The causes of the growth-degeneration pattern of the nucellus are unknown.
6. During the development of the megaspore mother cell into the mature embryo sac dramatic cellular ultrastructural changes take place. This probably signifies a “change of guards” during which the gametophyte is freed from part of the controls by the ultrastructural units of the mother plant, preparing the ground after fertilization for a new, genetically independent sporophyte.
7. Upon closer examination, the seemingly simple processes of fertilization and embryogenesis, as described in textbooks, turn out to be very complex and full of problems. Is the role each male nucleus plays preordained or is it left to chance which male nucleus goes where? What causes the degeneration of the synergids and of the vegetative nucleus, and what protects the other two male nuclei from a similar fate? Which ultrastructural organelles are carried by the generative nuclei into their respective receptor cells and what is their role in them? Why do zygotes in some species develop after fertilization immediately into an embryo whereas in other species the zygote remains dormant for some time? What causes the polarity of the egg cell which, after fertilization, divides into one developmentally most active apical cell (giving rise to the embryo) and into another “lazy” basal cell which develops into the suspensor of “unknown function?”
8. In the source-sink relationship between photosynthesizing organs and the maturing seed there is one point at which the photosynthates pass from symplast to apoplast to symplast. The mechanism involved is largely unknown as well as the effect which environmental conditions have on this transport.

     

The history of research on white-green variegated plants

Plinius the Elder (23–79 A.D.) noted the existence of variegated varieties of ivy. But only subsequent to the middle of the last century, when botanists detected chloroplasts and began to study their origin, did variegation of leaves begin to be investigated scientifically. The white, yellow, or yellow-green parts of the many species of variegated leaves contain either leucoplasts, plastids in various stages of degeneration leading to their complete disappearance, or plastids containing only carotene or xanthophyll. In some cases normal and abnormal plastids are found together in “mixed” cells. In some cases the variegation is temperature and/or light sensitive.     

Stomatal responses to changes in humidity in plants growing in the desert

Summary The stomata of plants growing in the Negev Desert, namely the stomata of the mesomorphic leaves of Prunus armeniaca, the xeromorphic stems of Hammada scoparia, and the succulent leaves of Zygophyllum dumosum, respond to changes in air humidity. Under dry air conditions diffusion resistance increases. Under moist air conditions diffusion resistance decreases. When the stomata close at low air humidity the water content of the apricot leaves increases. The stomata open at high air humidity in spite of a decrease in leaf water content. This excludes a reaction via the water potential in the leaf tissue and proves that the stomatal aperture has a direct response to the evaporative conditions in the atmosphere. In all species the response to air humidity is maintained over a period of many hours also when the soil is considerably dry. The response is higher in plants with poor water supply then in well watered plants. Thus for field conditions and for morphologically different types of photosynthesizing organs the results confirm former experiments carried out with isolated epidermal strips.     

The influence of the origin of the mother plant on yield and germination of their caryopses in Aegilops ovata

Summary Dispersal units of Aegilops ovata contain in their spikelets caryopses differing in morphology, weight and germinability according to their position in the spikelets. Plants originating from caryopses of the diverse orders were grown under various photo- and thermoperiodic conditions, and the caryopses harvested germinated under uniform conditions. Under all conditions tested, the relative difference in weight between the caryopses of the various orders was not affected. But plants grown under long days and/or low temperatures produced heavier caryopses than plants grown under short days and/or high temperatures respectively. There was a positive correlation between weight and germinability in caryopses of the various orders derived from one mother plant, and a reverse correlation in caryopses formed on mother plants grown under low and high temperatures. When mother plants arising from various orders of caryopses were grown under long days and low temperatures, they produced caryopses of different germinability, showing that the origin of the mother plant may affect the germination qualities of its offspring. The implications of these results are discussed.