GYMNOSPEEM^ 



177 



to form the suspensor takes place in the top cell as well as the 

 middle one. 



In the Cycadese the embrj^o is formed by a process of free- 

 cell formation leading to the construction of a mass of cells, the 

 lower part of which grows out into the tissue of the prothallium 

 forming a suspensor, at the end of which an embrj'o is developed. 

 Eiiliedra of the Gnetaceae behaves similarly, but each embryonic 

 cell grows out separately in the same way, and so a number of 

 embrj'os are formed ; in GmTigo the mass of cells formed in the 

 zygote constitutes the embryo, and there is no suspensor. Other 

 modifications occur in Wehvitschia and Gnetum. 



In most cases the zygote gives rise to a number of embryos ; 

 in the ripe seed, however, 



only one is usually present, ^^^' ^^^' 



the others perishing in the 

 course of development. 



The seed is always albu- 

 minous ; a good deal of the 

 prothallium persists, forming 

 an endosperm, and a certain 

 amount of the original nucel- 

 lus is also present, forming 

 perisperm. The embryo is 

 found embedded in the endo- 

 sperm : it consists of an axis, 

 with plumule and radicle, 

 and bears a variable number 

 of cotyledons, sometimes 

 only one, sometimes as many 

 as fifteen. The growing point of the root is peculiar, being 

 differentiated in the body of the embryo, some distance from its 

 lower end. The integument of the macrosporangium becomes 

 hard and woody and forms the testa of the seed. 



The macrospore remains in the sporangium or ovule aU the 

 time these changes are proceeding, and the sporangium does 

 not become detached from the sporophj^te from which it origi- 

 nated. The resulting structure is the seed whose composition 

 has just been described. In this body we have consequently 

 three generations represented. From the original sporophyte 

 are derived the testa, the perisperm or nucellus of the macro- 

 sporangium, and tlie embryo sac or macrospore. From tlie 

 gametophyte we have the endosperm or prothallium. From 

 the zygote, the commencement of the new sporophytic genera- 



VOL. II. N 



Fig. 929. Begiimiug of development of 

 embryo in Finns. The right-hand ovum 

 is a little the more advanced. 



