CEOSS-POLLINATION 519 



animals. From this point of view plants have been classed as 

 ayiemopliilous or wind-pollinated, hydroj^hiloiis or water- 

 pollinated, entomophilous or insect-pollinated, or zoo^liilous or 

 those pollinated by other animals. 



Of these methods of pollination the anemophilous and the 

 entomophilous are the most wide -spread. In the former case 

 certain structural features are associated with the mode of 

 transference of the pollen. It is produced in such flowers in 

 great abundance, is extremely light and dry, and in some cases 

 furnished with bladders to aid its transport. The receptive 

 organ is sometimes a bulky cone, the leaves of which are sepa- 

 rated from each other and from the common axis by spaces 

 into which the pollen may drop ; sometimes it is a much 

 divided or plumose stigma, often furnished with hairs, so that 

 pollen falling on it may be readily retained. The method is a 

 wasteful one and involves the production of a superabundance 

 of pollen. On the other hand anemophilous flowers are alwa^'s 

 inconspicuous and of a comparatively humble type. 



Flowers which are pollinated by insects are usually much 

 larger and more show}^ not infrequently possessing irregular 

 corollas, and are often very highly coloured and provided with 

 characteristic odours. Their perianths and sometimes their 

 sporophylls are highly modified to adapt them to the habits 

 of their insect visitors. As a further attraction to the latter 

 they usually produce honey in some part of the flower, in 

 such a situation as will lead to the removal of pollen by the 

 insect in its search for the attractive liquid. The pollen itself 

 also is often the object of the insect's visit. Many special 

 mechanisms to secure the removal of the pollen from the 

 microsporophyll and its deposition upon the stigma of another 

 flower are to be met with ; indeed, almost every Natural Order 

 shows some modification in this direction. The consideration of 

 them in detail, however, is beyond our present purpose. 



Something akin to cross-pollination occurs in Azolla as 

 already described ; the massulae or collections of microspores 

 being floated to the macrospores, to the floats of which the 

 glochidia attach them. 



These mechanical adaptations are, however, not the only 

 means we find to secure cross-pollination. There are peculiari- 

 ties connected with what we may call the receptivity of the 

 pistil for any particular pollen. Of these the most generally 

 occurring is dichogamy, or the maturing of the microsporophylls 

 and the macrosporophylls at different times. Two varieties of 



