CHAPTER IV. 



THE TISSUES OF THE STEM. 



The mature tissues of a Plant are not homogeneous as they are 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 Vascular and Fibrous System : this is embedded in the softer 

 Ground-Tissue ; and the whole is covered on the outside by a con- 

 tinuous 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 may give external 

 protection. 



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

 the shoot of a Flowering Plant either the firmer strands may be 

 dissected out by hand, or their position may be studied by means 

 of sections. It is only in large and herbaceous plants that the former 

 method is effective ; but it is well to cam 7 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 



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