CHAPTER IV 



SEEDS 



25. Seed structure. It will be well to recall just w r hat 

 a seed is and what it needs for germination. It is made up of 

 three things that must be considered : (1) the hard covering 

 (testa), (2) the young plant (embryo), and (3) the food supply 

 (Fig. 1). 



Testa. Seeds differ in the hardness and thickness of the 

 covering, and also in its permeability. Two things must 

 pass through the seed coat before germination can begin, 

 namely water and air (it is the oxygen of the air that is 

 needed) . Some seed coats 

 are impervious to water, 

 others to air, and others 

 to both, and in nature 

 they may lie in the soil 

 for a long time before 

 germination begins, await- 

 ing changes in the testa, 

 through decay or other- 

 wise, . that will permit the 

 entrance of water or oxy- 

 gen or both. For example, 

 the testa of the seeds of let- 

 tuce is so impervious that 



the seeds are often stored for two or three years before being 

 sold for planting, and throughout this period the germinating 

 power is probably diminishing. On the other hand, peas and 

 beans and corn germinate with great promptness. Me- 

 chanical means are being devised to remove the obstruction 



317 



FIG. 1. Seed of a violet : the right figure 

 shows the hard seed coat (testa) ; the left 

 figure is a section, showing the embryo sur- 

 rounded by the food supply (endosperm), 

 which in turn is surrounded by the testa. 

 After BAILLON. 



