INFLUENCE OF ENVIRONMENT 267 
leaves, and roots, and as soon as this stage was fully 
evolved, an entirely new type of plant structure had 
come into existence, which was destined to become the 
predominant type of the future, finally culminating in 
the great group of seed-bearing plants. 
It is among the latter that there are found the most 
remarkable and perfect adaptations to special condi- 
tions. Being mostly terrestrial plants, they show, 
when compared with the lower plants, which are for 
the most part aquatic, a much greater development of 
mechanical tissues, by which the stem and leaves may 
be supported. The most highly developed of these 
mechanical tissues is the wood of the vascular bundles, 
which forms the great part of the skeleton of the stems 
of trees and shrubs, and also the framework of the 
leaves. The vascular bundles are first met with in the 
ferns, but occur in all the higher plants. In some 
vascular plants, like most Monocotyledons, the wood 
is poorly developed and of little use as a support- 
ing tissue, and in these, as well as in many herba- 
ceous Dicotyledons, the mechanical tissues of the stem 
belong principally to the outer part of the ground-tissue, 
especially the layers of cells just below the epidermis. 
These are frequently provided with thickened walls so 
that the mechanical tissue forms a cylinder just below 
the epidermis, which is itself often furnished with 
thickened cell-walls. 
The woody tissue reaches its greatest development 
in the stems of those Conifers and Dicotyledons which 
increase in diameter from year to year owing to the 
presence of the so-called “open” vascular bundles, ¢.e. 
those in which there is a permanent zone of growing 
