22 VEGETABLE PHYSIOLOGY 
Yet a third reason may be given. The leaves are very 
frequently so placed that they extend outwards from 
the plant and lie nearly parallel to the surface of the ground. 
In this way they present their edges to the wind and offer 
as little obstacle as possible to its passage through the 
tree, so minimising the risk of being torn off when the force 
of the wind is strong. As the wind passes between them 
they are made to rise and fall, but they offer much less 
resistance to its force than they would if they were not 
flattened. 
Besides the plants which we have beeu examining there 
are other forms of terrestrial habits which possess only 
weak axes, quite incapable of supporting any great 
development of their shoot system. These obtain support 
by clinging in various ways and holding by various mechan- 
isms to other structures, such ag the trunks of trees, walls, 
&c. In some cases they develop accessory root systems 
from some part of their shoots, such roots, usually of small 
dimensions, penetrating their supports and so securing 
anchorage. 
The first indication of structural differentiation in the 
vegetative body of the plant is a change in the character 
of the exterior, which has for its 
object the protection of the plant 
from external injurious influences. 
This can be seen even among the 
seaweeds, simple as is generally the 
structure of members of this group. 
ee Fucus and its allies, which form part 
Twartus or Pelvis, Of the class of the brown Alge, have 
suowine Cuaracrer or their external cells much smaller, 
inne se a more closely put together, and 
generally much denser than the rest 
of their tissue (fig. 19). In the group 
of the Mosses certain arrangements of this kind can be 
seen. The common bog moss (Sphagnum) shows its stem 
to have on the outside several layers of large empty cells 
