164 PART III. PHYSIOLOGY. [ 40 



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

 of which they form part. 



The intercellular spaces (p. 89) of this tissue, which are es- 

 pecially large in submerged parts of aquatic plants, are of great 

 importance in connection with transpiration and the distribution 

 of gases in the plant : they communicate with the interior by 

 means of the stomata and the lenticels. 



c. The Sclerenchymatous Tissue (see p. 92), 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. 

 conidiophores 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 insuf- 

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

 (stereom) is developed, and it is distributed in the body in just 

 such a manner as most adequately meets the mechanical require- 

 ments in each particular case (p. 120, Fig. 98). Stereom is most 

 perfectly developed in the stems of land-plants which grow erect 

 and have to support the weight of many leaves and branches ; 

 whereas in water-plants the development of stereom is rudi- 

 mentary, for their stems, being supported by the water, do not 

 need to be highly rigid. 



When it is developed in the walls of fruits or in the seed-coats, 

 the sclerenchymatous tissue serves to protect the seed from being 

 eaten or digested by animals. 



d. The Tracheal Tissue of the Xylem (see p. 93). It is clear 

 that when a plant-body is massive, partly subterranean and partly 

 sub-aerial, there must be some means for readily distributing the 

 water and other substances absorbed by the root. This distribu- 

 tion may take place by diffusion from cell to cell ; and, as a matter 

 of fact, this mode of distribution suffices in some plants in which 

 the seat of absorption is not far from that of consumption (e.g. 

 larger Fungi and Algae). But when these points are widely sepa- 

 rated, special conducting-tissue, in the form of the tracheal tissue 

 of the xylem, is differentiated. 



The function of this tracheal tissue is demonstrated by the fol- 

 lowing experiment. If a cut be made all round the stem of a dico- 

 tyledonous tree, to such a depth as to penetrate far into the xylem, 



