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 (1)), 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 meinbers 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 
