IN THE SYSTEMATIC STUDY OF THE HIGHER PLANTS. 359 



size in different species ; and also in other genera of the family ; 

 and their relative frequency and form often afford useful specific 

 characters. The upper surface of the adult leaf is often rough 

 from the presence of the numerous cystoliths which occur as small 

 dots, or shorter or longer lines, or they may be curved. 



In the family Acanthaceae also the form varies in different 

 genera and species. [ 



I should exceed the time at my disposal, and weary you into 

 the bargain, if I went into the subject of plant- hairs — a beautiful 

 and interesting microscopic study. As regards their distribution 

 from the point of view of the systematist I may quote the apt. 

 remark of De Bary, that the case is similar to that of the forms, 

 of foliage-leaves. On the one hand there is great uniformity in 

 the majority of species and genera of one family, at least as- 

 regards one characteristic form of hair, such as for instance the 

 stiff hairs of the Borage family, the short capitate hairs and scales, 

 of the Dead-nettle family, the star- shaped hairs of the Crucifers, 

 the tufted hairs of the Mallows and the three characteristic forms 

 of hair of most of the Hawkweeds. On the other hand, a great 

 variety of form may occur in the same family or even genus ; or 

 one characteristic form recurs in corresponding parts in the most 

 diverse families or genera, such as the stinging-hairs of the nettle- 

 family and of the widely removed family Loasaceae. 



So far I have dealt with vegetative structure only, but the 

 special organs of the flower also supply minute characters whick 

 are useful for systematic purposes. I will refer briefly to one only, 

 viz. the characters of the pollen-grain. To some extent these are 

 obviously adaptive. In a few submerged aquatic plants, pollina- 

 tion, the transfer of the pollen from the stamen to the stigma of 

 the ovary occurs beneath the smiace of the water, and in these 

 cases the pollen sometimes takes the form of long threads. 



In many plants the pollen is distributed by aid of air-currents^ 

 and here the grains are very small and smooth, like fine dust, and 

 do not cling closely together ; in the pine the outer skin is blown 

 out to form a pair of air-bladders. 



In flowers in which the pollen is transferred on some part of 



the body of an insect which has visited the flower in search of 



honey or pollen, the grains are sticky and cling together in smal^ 



masses. In these cases there is an extraordinary variety cf 



f orm and external sculpturing of the grain, as to the meaning 



