391 



SIR J. LUBBOCK — PIirTOBIOLOGICAL OBSERYATIOXS. 



here other considerations come into play. The protectiou of the 

 embryo is mainly effected by the outer coverings ; but the pen- 



Fig. 130. 



Long and traiisverse sections of seed of DeJjjhlnlum Stajyhisagria, 



X 12. 7 



sperm itself contributes also *, and hence a small embryo is less 

 liable to injury. 



Size of Cotyledons, 



It is hardly necessary to say that the size of tbe plant does 

 not deteru)ine that of the cotyledons. Winkler has pointed 

 out that the largest of our Nettles has the smallest cotyledons f- 

 It is, on the other hand, natural that large seeds, as a general rule, 

 produce large cotyledons. This is, however, by no means a com- 

 plete explanation. There are many cases in which the cotyledons 

 grow considerably after quitting tlie seed. In the wonderful 

 genus JVelwitischia it was, at one time, supposed that the two 

 great leaves Avere persistent cotyledons, This view is now aban- 

 doned. In many of the Monocotyledons, however, the coty- 

 ledons acquire a considerable length, I have already had occasion 

 to allude to cases among the Dicotyledons in which the cotyledon 

 continues to grow for some time after quitting the seed. 



* See Marlotb, " Veber Mech. Scluitzmittel der Samen gegen schadliehe 

 Einflusse \oa Aus^^en/' Engler's Bot. Jahrb. 1SS3. 



t *'Ueberdie Keimbliitter der deutsehen Dicotylen/' Verb. bot. Ver. Bran- 

 denburg, 1874, p. 11. 



