96 BOTANY 
and air that goes on in the crowded forest. In the 
struggle for existence the plants best suited to the 
particular environment have ousted all others, so that 
the survival of the fittest has been imevitably accom- 
panied by the elimination of the unfit. 
Prostrate stems, though not capable of displaying 
their leaves to the best advantage, are, nevertheless, 
suited to certain conditions. On the windswept sand 
dunes along the coast, erect stems would be at the 
merey of every gale, and so from such places they have 
been ousted by creeping plants that press themselves 
close to the earth. There are other advantages in the 
prostrate habit besides that of escaping the wind. 
Creeping plants may root at every node, and _ by 
spreading over considerable areas derive nourishment- 
from a large body of soil. Furthermore, if a stem be 
injured or the soil be blown away from some of the 
roots, there is always the hkehhood that there will be 
left untouched some portion of the plant, still rooted 
in the soil and capable of carrying on its existence. 
Then, again, such plants, by covering the soil, protect 
it from the sun, and so reduce loss of water by evapora- 
tion. No wonder then that in such an environment they 
defy all competitors in the struggle for existence. The 
Silver Sand Grass (Spvufex hirsutus) a native sand- 
binding grass, and Calystegia soldanella, a New Zealand 
plant of the convolvulus family found on the sand 
dunes, are typical examples. Pennyroyal, a typical 
Huropean weed, having unfortunately been introduced 
into the Dominion, has, with its spreading prostrate 
stems taken charge of the damp meadows in the neigh- 
bourhood of swamps and streams. . | 
Climbing Stems.—It is in the forest that climbing 
stems are found in the greatest profusion. From year 
to year myriads of seeds must fall from the trees and 
germinate on the forest floor, and yet how rare for one 
to establish itself. It is impossible to produce, with 
