58 



LEAVES. 



the under surface ; the upper surface performiiig the func- 

 tions usually belonging to the under side. The Parenchyma 

 appears to the unassisted eye a mass of irregularly arranged 

 cells, bjt by careful examination, aided by the microscope, 

 we find a remarkable regularity in the arrangement of the 

 cells. If \vs take a thin slice, made by a vertical section of 

 the leaf of an Apple or Peach tree, and observe it by a good 

 magnifier we shall find tiiat immediately beneath the cuticle 

 which consists of a single row of cells, two or three layers of 

 cylindrical cells arranged perpendicularly to the surface, with 

 very small intercellular cavities. Between them and the 

 under surface are four or five rows of similar cells, but differ- 

 ently arranged touching each other by their ends and lying 

 inclined to the surface of the leaf forming comparatively 

 large cavities, particularly immediately beneath the stomates. 

 Fig. 34, a, exhibits a type of the arrangement of dicotyledons. 



3a 



That side of the leaf which is 

 furnished with stomates being 

 cavernous, and the opposite 

 side more compact. Those 

 leaves, which have the sto- 

 mates equally distributed on 

 boih surfaces, and those also 

 which have no stomates, have 

 cells of the parenchyma of 

 the two surfaces similarly ar- 

 ranged. Our space forbids 

 our recording here the numer- 

 ous discoveries of Mirbel, Mohl, Bronorniart 

 and others in this interesting department of 

 vegetable anatomy. The example above giv- 

 en will give the student an idea of the arrange- 

 ment made for the purposes of digestion, res- 

 piration and perspiration in the plant, which functions we 

 shall notice in another place. 



^^. The veins of the leaf, which ramify in every direction 

 throusfh the Parenchyma, are composed as we before obser- 

 ved, of vessels enclosed by a sheath of woody fibre. These 

 veins serve two purposes, that of giving form and support to 

 the Parenchyma and aff*ordincr channels for the circulation of 

 the sap to the various parts of the leaf, and returning it to the 

 stem. The veins are largest where they enter the leaf, and 

 decrease as they proceed and ramify till they are lost to our 

 observation in the cellular tissue. There seems to be two 

 separate venous systems in the leaf, one over the other con- 



