L£AVE8. 59 



nected bv the extremities of the veins, the upper one being the 

 syst'^in, through wliich tlic sap passes into the leaf, and the 

 lowor conveys it after elahoratinn hack into the stem. It is 

 genernllv diti'icult anil often impossible to distinguish these 

 two systems ; hut l)y maceration, some leaves will scj)arate 

 into two lamina. It is recorded of an East India plant, that 

 the cohesion of the two plates is so slight, that the lamina 

 mnv be easily separated and the hand inserted between the 

 furt'aces, as in a glove. 



0(5. The structure of the leaf seems to have been designed 

 in direct reference to the function of elaborating the sap by 

 eliminating a part of the water and exposing the whole to (he 

 action of the air. These processes are provided ibr and reg- 

 ulated in the most beautiful manner in the anatomy of the 

 leaf. The leaves of trees have a general position, nearly par- 

 allel with the earth. The upper side, exposed to the direct 

 action of the sun's rays, lest evaporation should take place 

 too rapidly, under such circumstances, has few or no pores 

 and the cells being of a cylindrical form are arranged with 

 their ends towards the surface, thus presenting the least sur- 

 face of each cell to the influence of the solar rays, by this 

 arrangement linuting their influence. Here we observe 

 three precautions taken evidently to prevent excessive evap- 

 oration. Again, those leaves whose sides are equally exposed 

 to the action of the sun's rays, are equally furnished with 

 these evaporating pores ; the numher in this case determining 

 the evaporation. In leaves floating upon the surface of the 

 water, it is evident, that pores on the under surface would avail 

 nothing in evaporation, and perhaps be destructive U) the or- 

 gan ; hut the up|)er surface in this case, is furnished with nu- 

 merous exceedingly miriute pores connected with deep nar- 

 row cavities next the surface, and these are connected with 

 larger ones iVi the interior so tliat by this arrangement 

 evaporation can take place, but slowly. Nature using there 

 precai'tions wh<;re she seems obliged to use the upper surface 

 to perform an indispensable function, which without such 

 precaution, would endanger the safety of the plant. 



65. Forms of Leaves — By the arrangement and develop- 

 ment of the vencius and paienchymous systems of leaves, eve- 

 ry variety of form.uh ch leaves assume, may be reduced to a 

 few very sim[le piincip!es. 



1. Fork-veined leaves are those leaves in ^\hich the jirima- 

 ry veins divide into two, nearly tqual, secondary veins, form- 

 ing a fork, and these sub-divicied in the same manner. The 



