THE SEED 325 



after the fertilisation of the egg cell, the development of the 

 new sporophyte starts at once and proceeds until an embryo 

 is formed ; development then ceases temporarily and the 

 embryo loses water, while changes in the integuments 

 of the ovule result in its enclosure in a more or less resistant 

 envelope. Thus is formed the seed. The seed typically 

 contains a store of food, fat or carbohydrates, and proteins. 

 These may be stored in the cotyledons, in the hypocotyl, 

 or in a special storage tissue. The storage tissue may be 

 derived from the nucellus of the ovule, the megasporangium, 

 when it is called the perisperm, a rather uncommon case 

 of which the pepper is an example. Most frequently it 

 develops as the result of a second fusion occurring in the 

 embryo sac, in which three nuclei are concerned ; one of 

 these is a sister of the egg cell, the second is one of the group 

 of four antipodal cells, which may be regarded as the last 

 vestiges of the vegetative part of the female gametophyte ; 

 the third is the second sperm cell which enters from the 

 pollen tube. The triple fusion nucleus divides many times, 

 and after cell formation has taken place the nutritive tissue 

 known as the endosperm is formed. It is well shown in 

 the cereals. The food store of the gymnosperm seed is an 

 endosperm derived from gametophytic tissue which is well 

 developed within the embryo sac. 



The seed is thus a body of complex structure and of 

 multiple origin. The seed coat belongs to the parent 

 sporophyte, as does the perisperm if present. The endo- 

 sperm is gametophytic in origin in the gymnosperms, while 

 in the angiosperms it is a unique body in a sense homo- 

 logous with the new sporophyte. The embryo is the new 

 sporophyte generation. The whole may thus include three 

 different generations in its constitution. It is not a repro- 

 ductive body in any strict sense of the v/ord. The repro- 

 ductive bodies, in the seed plants as in others, are the spores 

 and the gametes. The seed is the consequence of two 

 separate reproductive events, and is, in fact, simply a young 

 plant in which development has been arrested, and which 

 has at this stage been cast loose from the parent, enclosed 



