192 



MECHANICAL SYSTEM 



the margins, which are of the same thickness as the rest of leaf. 

 Each pinna thus has two straight edges, the height of which exactly 

 equals the thickness of the leaf ; from the middle of either edge there 

 projects a narrow strip of dried-up tissue (Fig. 67 D, z). This strip, 

 which is approximately conical in cross-section, is a relic of the tissue 

 that formerly connected two adjacent leaflets, the pinnate condition 

 being here as is well known due to the splitting up of a primarily 



continuous lamina. In the angle 

 contained by the edge of the pinna 

 on the one hand, and by its upper 

 or lower surface on the other, there 

 is situated a single large bundle 

 of fibres. In this way the strands 

 which form the two most powerful 

 subepidermal girders of the whole 

 pinna serve at the same time for the 

 special protection of the margin. 



The marginal fibrous strands of 

 leaves are, however, not always strictly 

 subepidermal ; they may be separated 

 from the epidermis by one or two 

 layers of green parenchyma, in which 

 case it is usual for the marginal 

 mestome-bundle to abut against one 

 side of the fibrous strand or to be 

 embedded within the latter (Theo- 

 phrasta imperialis, Fig. 67 b). Finally, 

 it is not at all uncommon for other 

 subepidermal strands to unite with 

 the previously strengthened marginal 

 veins, in which case, of course, a great 

 variety of cross-sectional outlines may result. 



The strengthening of the leaf-margin is effected in a very remark- 

 able manner in the genus Aloe, where the requisite special protection is 

 afforded by a subepidermal layer of palisade-shaped sclerenchymatous 

 cells. Towards the flat surface of the leaf this layer passes over 

 gradually into ordinary photosynthetic palisade-tissue. There seems, 

 therefore, no doubt, that in Aloe the need for special protection of the 

 leaf-margin has forced the marginal photosynthetic cells to give up 

 their original function, and to become converted into mechanical 

 elements. 



Where the leaf-margin is entire, the arrangements for its special 

 protection are very much the same at every point. In the case of 



Fio. 68. 



T.S. across one of the commissures of a 

 leaf-sheath of Equisetum li.iema.te. A. Meri- 

 stematic condition. B. Adult state. 



