48 LEAVES. 



two systems ; but by maceration some leaves will separate into 

 two laminae. It is recorded of an East India plant, that the 

 cohesion of the two plates is so slight that the laminae may be 

 easily separated, and the hand inserted between the surfaces, as 

 in a glove. 



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

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

 tion 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 

 toward the surface, thus presenting the least surface of each cell 

 to the influence of the solar rays, by this arrangement limiting 

 their influence. Here we observe three precautions taken, 

 evidently to prevent excessive evaporation. Again, those leaves 

 whose sides are equally exposed to the action of the sun's rays, 

 are equally furnished with these evaporating pores ; the num- 

 ber in this case determining the evaporation. In leaves float- 

 ing upon the surface of the water, it is evident that pores on the 

 under surface would avail nothing in evaporation, and perhaps 

 be destructive to the organ ; but the upper surface, in this case, 

 is furnished with numerous exceedingly minute pores connected 

 with deep narrow cavities next the surface, and these are con- 

 nected with larger ones in the interior ; so that by this arrange- 

 ment evaporation can take place but slowly ; Nature using these 

 precautions where she seems obliged to use the upper surface to 

 perform an indispensable function, which without such precau- 

 tion would endanger the safety of the plant. Leaves developed 

 under water have no stomata, and no fibro-vascular system, 

 but consist entirely of cells, forming cavities in their interior 

 which are filled with air to float the leaf. 



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

 ment of the veins and parenchymous system of leaves, every 

 variety of form which leaves assume may be reduced to three 

 varieties. 



(1.) Fork-veined leaves are those in which the primary 

 veins divide into two nearly equal secondary veins, forming a 

 fork, and these subdivided in the same manner. The veins 

 always proceed directly from their origin to the margin of the 

 leaf, without forming any meshes or network, as is exhibited 



May they be separated I In what plant especially 9 — 86. What is the 

 position of the leaves of trees I What prevents excessive evaporation 

 from the upper surfaces 1 How with perpendicular leavea 1 Those rloat- 

 ing in water? Leaves uinlcr water ( — 87. How many varieties of forms 

 of leaves i What are fork-veined leaves ? 



