THE ANGIOSPERMAE 



1435 



13 19). Accounts of the penetration of the papilla cells are ill-founded, 

 though the young tubes may cling to them and even wind round them. On 

 the other hand some stigmas have a continuous covering of cuticle and this 

 the tubes perforate probably enzymatically, leaving round holes where 

 they have gone through. Many stigmas are covered by a profuse secretion 

 containing mucilage and oils, in which the pollen grains are bathed. This 

 does not seem to be chemically important but rather to be a protection 

 against desiccation. 



Fig. 13 19. — Eschscholtzia calif ornica. Germination 

 of pollen grains on the stigma, showing pene- 

 tration of the pollen tubes into the stigmatic 

 tissue. 



Under most circumstances the conditions of germination on the stigma 

 seem to be fairly uniform and pollen will readily germinate on the stigmas 

 of plants which are systematically remote {e.g., Lathyrus pollen on Comal- 

 laria). Recondite reactions can, however, be called out by hybrid pollina- 

 tion and intra-specific incompatibility or even by autogamy ; reactions which 

 are presumably in abeyance under usual conditions. These reactions re- 

 semble those of pathological immunity and they are of various degrees of 



