194 MECHANICAL SYSTEM 



Cross-ties of this kind are particularly well-developed in certain 

 species of Ribes (e.g. R. mlrum, Fig. 67 F, gg). In other instances, 

 the base of each indentation is embraced by a sickle-shaped strand 

 composed of strongly thickened usually collenchymatous cells. Such 

 strands, composed of thick-walled colourless cells elongated in a direction 

 parallel to the margin, are well seen in species of Poterium and 

 Sangwisorba (Fig. 67 e), where they contrast very sharply with the 

 adjoining photosynthetic parenchyma. These local mechanical strands 

 often abut internally against one of the above-described tangential 

 mestome-anastomoses. 105 



In the leaves of many Ferns, each indentation of the margin is 

 protected by a whole series of very thick-walled crescentic stereides ; 

 Asplenium diver sifolium affords an excellent illustration of this remark- 

 able device for the local strengthening of the leaf-margin (Fig. 69). 

 In this connection mention must also be made of the peculiar "anchor- 

 cells " of Equisetum, which were first described by C. Midler (Fig. 68); 

 these help to prevent the leaf-sheath from tearing along the relatively 

 weak " commissures " {i.e. between the successive teeth). In view of 

 their function these peculiar mechanical elements might more aptly 

 be termed " clarnp-cells," since they serve as clamps for holding 

 together adjacent segments of the sheath. 106 



V. THE INFLUENCE OF EXTERNAL FACTORS UPON THE 

 DEVELOPMENT OF THE MECHANICAL SYSTEM. 



Generally speaking, botli the qualitative and the quantitative 

 development of the mechanical system are included among the heredi- 

 tary characters of the species. Nevertheless a certain amount of direct 

 accommodation to external conditions on the part of the mechanical 

 system may take place during the life of the individual, or in the 

 course of development of some of its organs. 



In this connection special stress must be laid upon the relations 

 between the quantitative development of the mechanical system and 

 the action of those mechanical forces which are ultimately responsible 

 for the presence of the entire skeletal system. 107 It is probable that 

 the forces in question act as stimuli that tend to accentuate the 

 qualitative and quantitative development of the stereome, at any rate 

 so long as they do not exceed certain limits of intensity. Certain 

 facts bearing on this point were recorded over a hundred years ago by 

 Knight. A number of young [seedling] Apple-trees, with stems from 

 six to seven feet in height [between the roots and the first branches], 

 were tied to stakes in such a manner that the lower portions of the 

 stems were nearly deprived of all motion to the height of three feet 



