676 PART IV. — THE PHYSIOLOGY OF PLANTS. 



h. The ParencJiymatoti.^ Tissue (see p. 131), consisting as it 

 typically does of cells which contain living protoplasm, is the 

 seat, not only of the metabolic processes, but also of the movements 

 and irritability of plants. 



Different nutritive functions are discharged by various regions 

 of this tissue. For instance, the pai'enchymatous tissue of sub- 

 aerial parts, lying near the surface and exposed to light, contains 

 chlorophyll, and carries on the assimilation of carbon : this applies 

 especially to the leaves. Again, the cells of this tissue are 

 frequently glandular (see p. 137), containing or excreting various 

 Avaste-products : or they serve as depositoi'ies of reserve plastic 

 substances (e.g. starch, etc.), or as conducting-tissue for organic 

 substances. 



It may also be pointed out that this tissue is connected with 

 the reproductive processes in that the reproductive organs (e.g. 

 hypodermal archesporium of most plants) are developed from it 

 in part. 



. 1 Tirther, the cells of this tissue, having usually extensible 

 walls, are capable of being turgid and of varying in bulk : hence 

 they are the seat of the movements of those members, or parts of 

 them, in which movement is a mechanical possibility ; and when 

 turgid, they give a considerable degree of rigidity to the member 

 of which they form part. 



The intercellular spaces of this tissue, which are especially 

 large in submerged parts of aquatic plants, are of great im- 

 portance in connexion with transpiration and the distribution of 

 gases in the plant : they communicate with the exterior by means 

 of the stomata and the lenticels. 



c. The Sclerenchymatous Tissue (see p. 132), more especially the 

 prosenchymatous or fibrous form of it, has the purely mechanical 

 function of giving firmness to the members in which it is present. 

 Whilst it is true that a considerable degree of rigidity is afforded 

 by turgid parenchymatous tissue, and that many members con- 

 taining little or no sclerenchymatous tissue can grow erect (e.g. 

 gonidiophores of moulds, and succulent stems of herbaceous 

 annuals), yet this source of rigidity is precarious, as it is so largely 

 dependent upon external conditions, and is therefore insufficient 

 in the case of perennial plants. In these plants rigid tissue 

 (stereom ; see p. 133) is developed, and it is distributed in the 

 body in just such a manner as most adequately meets the 

 mechanical requirements in each particular case (p. 170). Stereom 



