CHAPTER III, 



THE TISSUES OF THE STEM. 



The mature tissues of a Plant are not homogeneous as in the apical 

 bud. At first they are all soft. But as they pass over to the mature 

 condition, while certain tissues retain their relatively thin-walls, 

 others become indurated, forming strands which are mechanically 

 resistant. This is illustrated in a familiar way in the shoots of garden 

 vegetables. If these are allowed to mature too far they become 

 stringy, owing to the development of toughened strands. Where 

 the succulent tissues preponderate the harder strands form isolated 

 threads embedded in the softer tissues. In other cases they may be 

 fused into larger tracts, and this is especially so as they grow older. 

 They thus form in tree trunks and twigs of woody plants a cylindrical 

 core, from which smaller strands extend outwards into the leaves and 

 branches. The mature Shoot, with its constituent axis and leaves, 

 is thus composed of a relatively firm skeleton, consisting of the 

 Fibrous and Vascular System : this is embedded in the softer Ground- 

 Tissue ; and the whole is covered on the outside by a continuous skin 

 of the Epidermis. The first of these serves for conduction, and gives 

 mechanical strength : the second carries on the functions of nutrition 

 and storage : while the epidermal system gives external protection. 



In order to obtain a more exact idea of the general construction of 

 the shoot of a Flowering Plant it is possible either to dissect out the 

 firmer strands by hand, or to study their position by sections. It is 

 only in large and herbaceous plants that the former method is 

 effective ; but it is well to carry it out in some such plant as the 

 Sunflower, for this gives a better understanding of the results obtained 

 by sections. By either method it is possible to trace the course and 

 connections of the strands through the softer tissues, and so to construct 

 the vascular skeleton of the shoot. Such a skeleton is shown for 



B.B. 33 c 



