442 VEGETABLE PHYSIOLOGY 



their pollen. The second condition is known as protogyny, 

 and is the converse of the first, the stigma withering before 

 the pollen is mature. This condition occurs in both anemo- 

 philous and entomophilous flowers ; certain of the Plantains 

 (Plantago) and some grasses (Anthoxanthum, &c.) show it 

 in the former group, as does Scrophularia among the latter. 



Something corresponding to dichogamy is found among 

 the Ferns, where the antheridia and archegonia on a pro- 

 thallium do not mature simultaneously. Cross -fertilisation 

 must consequently be the only form possible. The same 

 peculiarity may be observed among the Mosses. 



Another means often observed to secure cross-pollination 

 is diclinism, or the production of the stamens and carpels 

 in different flowers. Diclinous plants may be monoecious, 

 where the staminate and pistillate flowers are on the same 

 plant ; dioecious, where they are on different plants ; or poly- 

 gamous, where a plant bears flowers with stamens and 

 carpels as well as others which contain only one or the 

 other kind of sporophyll. 



The terms ' monoecious ' and ' dioecious ' are sometimes 

 applied to the Cryptogams, when their sexual organs are 

 upon the same or upon different plants. They then refer, 

 of course, to the gametophytic and not to the sporophytic 

 phase of the life cycle as in the cases just quoted. 



Some flowers exhibit a peculiarity of form, which is 

 an adaptation favouring cross-pollination. The plants 

 possess flowers of two kinds, which are specially related 

 to each other. The most familiar instance in our own 

 flora is the common Primrose, which has five stamens and 

 a club-shaped stigma. In some flowers the stigma is 

 placed just in the throat of the corolla, and the stamens 

 some little way down its tube. In the rest of the flowers 

 the positions are reversed. We have here an adaptation 

 to the visiting insect, for when it touches the stamens of a 

 short-styled form, it covers with pollen the part of its body 

 which will come into contact with the stigma of the next 

 long- styled flower it alights upon. Another portion of its 



