1572 A TEXTBOOK OF THEORETICAL BOTANY 



GERMINATION AND SEEDLINGS 



The primary fact in the process of germination is the swelling of the 

 seed by the absorption of water. The content of water rises from the 

 5-T0 per cent., which is characteristic of air-dry seeds, to 30-50 per cent, 

 or more in some cases. There is great variability in the amount of water 

 absorbed, not only as between different species but also in the same species 

 under varying conditions, e.g., temperature, oxygen supply, etc. Starchy 

 seeds generally absorb more than seeds with a high protein content, though 

 they may absorb it more slowly, and most seeds absorb less when immersed 

 than when there is a free supply of air. An exception to this is the seed of 

 Orysa, which is a water plant. Indeed there is evidence of an ecological 

 differentiation in regard to water absorption, seeds of dry environments 

 generally germinating at a lower water content than those accustomed to 

 mesophytic or hydrophytic conditions. As the seed is not a homogeneous 

 structure, the rate of water uptake depends on the differing powers of 

 imbibition of the testa, the endosperm and the embryo respectively and it 

 may be irregular and slow. Some of the water absorption by the endosperm 

 and embryo is osmotic, since it has been shown that in the dormant con- 

 dition the cells of these structures are plasmolysed, but become turgid 

 before germination. 



We have seen previously that the testas of many seeds are almost 

 completely impermeable and that water uptake and germination may be 

 indefinitely delayed until the testa has been broken or has decayed. Even 

 with free penetration of water it may be upwards of 48 hours before the 

 seed becomes fully imbibed. This absorption of water is accompanied by 

 considerable imbibitional swelling, which may burst the testa and, as we 

 shall see hereafter, may also cause the bursting of hard fruit shells in 

 which the seeds are enclosed. It is the necessary first condition for 

 germination. 



A point which is important is that many seeds absorb water more 

 readily at low temperatures and some indeed, e.g., Papaver rhoeas, will not 

 germinate at temperatures above 15"" C. This inhibits the germination of 

 the seeds during the summer, which restricts this species to one generation 

 a year, although the vegetative period is relatively short. 



We have already referred, in the earlier part of this chapter, to some of 

 the conditions of delayed germination and to the very complex nature of 

 the seed coats. Many testas contain cell layers, or cell walls of a single 

 layer, which produce amyloid or mucilage with strong affinities for water, 

 which is readily absorbed, while others have no such substances and may 

 offer the greatest resistance to penetration. To the latter class belong many 

 of the seeds formed in berries, which, bathed as they are by nutritive, 

 albeit somewhat concentrated juices, would otherwise tend to germinate 

 in the fruit, to their detriment. Chemical inhibitions also play a part in 

 restraining germination in many cases, especially in fleshy fruits. 



It is not our intention to enter now into the problems of the physiology 



