SECT. II PHYSIOLOGY 265 



resistance to thu external pressure is intensified. It' the resistance of the body 

 exerting the pressure cannot be overcome, the plasticity of the cell walls renders 

 possible a most intimate contact with it ; thus, for instance, roots and root- 

 hairs which penetrate a narrow cavity fill it so completely that they seem to have 

 been poured into it in a fluid state. It would be natural to suppose that the effect 

 of such a tractive force as a pull Mould accelerate growth in length by aiding and 

 maintaining turgor expansion. But the regulative control exercised by the jjroto- 

 plasm over the processes of growth is such that mechanical strain, as Hegler has 

 shown, first acts upon growth to retard it, but then causes an acceleration of even 

 20 per cent (™). 



III. The Internal Construction of Organs 



The internal development of an organ commences close behind the 

 growing point and lasts for a longer or shorter time. While hairs are 

 frec[uently very rapidly fully developed the definite form and structure 

 of the internal tissues is often only completed after the phase of 

 elongation is ended. When secondary growth in thickness takes place 

 there is no termination to the internal development. The development 

 of the " permanent tissues " from the primary and secondary meristems 

 has been described in the morphological section. Here it is only 

 necessary to recall the fact that the following processes are con- 

 cerned ; in the first place the formation of cells by cell-division which 

 takes place in the embryonic tissues and at the commencement of 

 elongation ; following on this the separation of cells which gives rise 

 to intercellular spaces ; the inde})endent growth of the individual cells; 

 thickening and chemical changes of the cell-walls ; modifications (and 

 eventually in some cases the complete disappearance) of the cell- 

 contents ; and lastly fusions of cells (cf. p. 97). 



All the tissues cannot be referred to tlie primary growing [loints ; many arise 

 from the cambium, though this does not involve any essential distinction from 

 primary tissues : other tissues are formed in processes of regeneration ("^). In the 

 case of regeneration (p. 252), full-grown iiarenchymatous cells which normally have 

 ceased to be capable of growth may take part ; they may give rise to almost all the 

 tissues which are produced by the growing point. Thus, for example, when the 

 conducting tracts are interrupted, new vessels, M'hich restoi'e the connection, may be 

 formed from the jiareucliyma. It is true that the separated or interruiited tissues 

 are not always formed again, but substitutionary tissues may be developed. Thus, 

 as a rule, the epidermis is replaced by cork and only exceptionally is a true 

 epidermis witli stomata developed. On the callus wliich is so frequently formed in 

 regeneration cf. p. 162. 



In the arrangement of the tissues the same symmetry, which is 

 apparent in the external form of the organs of the plant, is seen ; the 

 construction of the organs is thus radial, bilateral, or dorsiventral. 

 When it is possible to experimentally modify the external symmetry 

 the internal structure is modified at the same time. A close connection 

 exists between external form and internal structure, and every change 



