THE LEAVES OF PLANTS. 



95 



of several layers of somewhat large and thickened cells, which have the power ot 



remaining distended, even when examined under the microscope a quality which, in 



some instances, is assisted by the presence of a 



spiral fibre within each cell (Fig. 44). These 



cells also very freely commuiacate with each k '^-' -'- -^-'-l 



other (Fig. 152). It may be fairly questioned 



if this is not a peculiar structure, since it is 



admitted that the cell-walls are not perfect, 



and it is certain that they do not collapse when 



cut open so readily as other cells. 



The connexions of the parenchyma are the 

 cuticular cells above and below, and the veins 

 of the leaves which pass through it. It is also, 

 unlike all other cells, exposed to the direct ac- 

 tion of the atmosphere, since it receives air 

 through the stomata, which have their cham- 

 bers within its structure. 



The functions of the parenchyma is to re- 

 ceive the juices from the upper layer of veins, 

 and, by the exposure of them to the atmosphere 

 and other influences, to elaborate them, and 

 thus to yield them up to the under or recumbent 

 set of vessels, to be returned to the stem of the 

 plant. It is therefore somewhat the ana- 

 logue of both the lungs and the stomach in 



animals, for it performs the functions of both these organs. All the functions of res- 

 piration, which are attributed to the stomata, are fairly due to the parenchyma of 

 leaves ; for the former bear to the latter only the relation which the mouth and wind- 

 pipe do to the lungs. The parenchyma, in common with the cuticular cells, is usually 

 of a green colour, from the presence of starch and chlorophyle within the cells, 



Forms of Leaves. The form of leaves is very varied, 

 but is permanent in the same species, and is conse- 

 quently the result of design. Except in a few instances 

 the leaf is never so far modified in form as that the 

 functions of respiration and digestion are interfered with,, 

 and therefore the precise neces- 

 sity for the infinite variations is 

 not clear, except as evidence of 

 that Creator's goodness which 

 cares for the beauty as well as the 

 utility of his works. 



The shape or outline of the leaf 

 depends on, or is modified by, the 

 length and relative position of the 

 veins. When the midrib divides 

 into branches, and when all the Pip. 154. Leaf of Hy- 

 branches diverge in the same plane, drocot y le vul * aris - 

 the leaf is flat, and this may be called the normal state of leayes (Fig. 153) ; when 



Kg. 152. A vertical section of the leaf of 

 the Euonymus japonicus, exhibiting the 

 cuticular andpurenchymatous structures. 



a, the cuticular cells of the upper sur- 

 face. 



b, the cuticular cells of the lower sur- 

 face, with stomata at c. 



e, the lower open parenchyma, with the 

 air chambers of the stomata, at d. 



Pig. 153. Elm Leaf. 



