520 MANUAL OF BOTANY 



this condition are met with ; in the first, known as protandry, 

 the stamens with their pollen are mature while the stigma is 

 still too little developed to be pollinated. Examples may be 

 fomid in tile Gentianaceae, Onagraceae, Campannlaceae, Compo- 

 sitae, &c. In Parnassia the receptive surface of the stigma is not 

 even formed until the anthers have discharged their pollen. The 

 second condition is called j^^'oi^rogynij, 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 {Plcmtago) and some grasses 

 {AniJioxanthum, &c.) show it in the former group, as does 

 Scropliularia among the latter. 



Something corresponding to dichogamy is found among the 

 Ferns, where the antheridia and archegonia on a prothallium 

 do not mature simultaneously. Cross-fertilisation must con- 

 sequently be the only form possible. The same peculiarity may 

 be observed among the Mosses. 



Another means often observed to secure cross-pollination is 

 dicUnism, 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 hermaphrodite flowers as well as either 

 staminate or pistillate ones. 



The same terms ' monoecious ' and ' dicecious ' are often applied 

 to the Cryptogams, when their sexual organs are upon the same 

 or different plants. They then refer, of course, to the game- 

 tophj^te and not the sporophyte phase as in the cases just 

 quoted. 



Some flowers show a peculiarity in form, which is sometimes 

 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 

 Prinn'ose, 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 

 remaining 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 which will 

 come in contact with the stigma of the next long- styled flower 

 it alights upon. The best seeds are produced when each 

 stigma is supplied with poUen from stamens occupying a corre- 

 sponding position. 



