74 



COMPOUND ORGANS OF PLANTS. 



culi or costas which proceed from the sides of the midrib, each 

 fasciculus consisting of ■woody fibre enclosing spiral vessels 

 throughout all its ramifications. 



The cellular system of the leaf. — This substance forms its 

 principal part, filling up the meshes in the network, formed by 

 the vascular system. To the naked eye it appears as a struc- 

 tureless pulpy mass of a green color, called parenchyma (n:apo, 

 beside or between, and ^tf^Jj"") anything efi'used or spread out.) 

 Under the microscope the parenchyma of the leaf no longer 

 appears as an unformed mass, but as a beautiful and regular 

 arrangement of cells, which are so disposed as most efiectively 

 to subserve those purposes of nutrition for which the leaf is 

 formed. 



In all leaves which present one surface to the sky and the 

 other to the ground, there is between the upper and under 

 cuticle two strata of parenchyma difierently arranged. In the 

 upper stratum of parenchyma, the cells are arranged in one or 

 more compact layers, vertically, or at right angles to the upper 

 surface of the leaf, so that they present the least possible 



Fig. 13. 



Y\%. 13. Magnified "view of the edge of a leaf. The parenchymals alone represented, 

 the woody ti^sui; being left out. a and h, show the epidermis and denser parenchyma 

 of the upper surface of tho leaf; Cj (Z, the looser parenchyma and epidermis cf ita 

 lower surface. 



