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, i.e. 

 those in which there is a permanent zone of growing 



