Green Leaves at Work 109 



square inch. In land-plants they are most numer- 

 ous on the lower or shadowed side of the leaf, 

 where moisture can not be drawn through them 

 too fast by the ardent rays of the sun. But the 

 floating leaves of water-plants have all their sto- 

 mata on their upper surfaces, which alone come into 

 contact with the air, and leaves which grow under 

 water have no stomata at all. 



Beach and desert plants must live between glar- 

 ing skies and parching sands. So, whatever their 

 more favored relatives do, these plants develop 

 succulent leaves. Such foliage is born by the South 

 African groundsel (Fig. 22 (i)), which has so adapted 

 itself to circumstances that it is singularly unlike 

 the too-familiar groundsel invading our gardens. 



The moisture which fate vouchsafes such plants 

 must be treasured for times of need, not drawn 

 speedily away by high winds or scorching sun. So 

 the stomata in their leaves are very few, and the leaf- 

 skin is thick and tough, so that vapor may not 

 exude through it. 



The cactus family has a few representatives 

 which grow wild as far north as Nantucket, but 

 most of its members live in the hottest situations 

 in tropic or semi-tropic lands. In such localities 

 there is danger that the plant's juices be scorched or 



