452 VEGETABLE PHYSIOLOGY 



may be found in the Gentianacese, Onagraceae, Campanu- 

 laceae, Composite, &c. In Parnassia the receptive surface 

 of the stigma is not even formed until the anthers have 

 discharged 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 anemophilous 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 observed in many cases 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 polygamous, 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 oar 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 



