202 



that's it; 



leaves of mosses there are only 

 veinlets, too small to be traced ; 

 and in thick fleshy leaves, like 

 those of the aloe, the vessels are 

 concealed, and therefore called 

 hidden veins.* 



Leaves spread out their broad surfaces in the 

 air for the same purpose, precisely, as these for 

 which the roots diffuse their fibres through the 

 soil ; the only difference being, that, while the 

 roots suck in chiefly liquid, the leaves inhale 

 almost wholly gaseous food. In the day time, 

 whether in the sunshine or in the shade, the 

 green leaves are constantly absorbing carbonic 

 acid gas from the air, and giving off oxygen 

 gas ; that is to say, they are continually appro- 

 priating carbon from the air. When night comes 

 this process is reversed, and they begin to absorb 

 oxygen, and give off carbonic acid gas. But the 

 latter process does not go on so rapidly as the < 

 former; so that, on the whole, plants when 

 growing gain a larger portion of carbon from 

 the air.f 



It has been found by experiment that the 

 leaves of herbs absorb moisture chiefly by their 

 upper surfaces, and the leaves of trees by 

 their under surfaces. The lower surfaces of the 

 leaves of trees appear to be endowed with the 

 greater capacity of absorbing moisture, chiefly 

 for the purpose of catching the ascending ex- 

 halations which must necessarily come in con- 

 tact with them as they rise, but which might 

 possibly escape, if absorbable only by the upper 

 surface, owing to the increased rapidity of their 

 ascent at an increased elevation; while the 

 upper surfaces of the leaves of herbs are en- 

 dowed with a greater absorbing power, owing 

 to their low stature, and to the slow ascent of 

 exhalations near the earth. 



The venation of exogenous 

 plants, 10, differs materially from 

 that of the endogenous, 10a. In 

 these the veins run 

 in nearly parallel 

 lines. The leaf- 

 stalk, 11, divides 

 into the primary 

 wins, 12, and be- 

 tween these lie the 

 veinlets, 13. There 

 are other arrange- 

 /j ments of veins, as 

 the equal veined in 

 ferns, the netted, the 

 5*4. curve-veined, the ra- 



• Orr*8 Circle of the Sciences. 



tF. W J huston's Agricultural Chemistry.— It is 

 How considered doubtful whether plants actually evolve 

 Carbonic acid gas by night. 



10a 



diating, the feather-like y &c. These 

 varieties impart interest to the 

 examination and comparison of 

 leaves, whether found in the 

 wood or the hedgerow, in the low 

 marsh, or upon the lofty hill. 



In every leaf there is a double 

 set of vessels : one by which the 

 sap is carried to the leaf, and 

 another by which it is removed 

 from those organs. The former 

 occupies the upper, and the latter 

 the under surface. But an ob- 

 server could not detect the double 

 set of vessels, because they are 

 united to each other, and pre- 

 sent a single appearance. When 

 leaves are decayed, however, the 

 two systems of vessels may be 

 separated with care.* 



Sometimes the body of the 

 leaf is attached immediately to 

 the stem, without any inter- 

 mediate stalk, as in the common 

 poppy, 14 ; such leaves are said 

 to be sessile. More frequently, 



16 



565. 



however, there is an intervening 

 petiole or leaf-stalk, 15, and 

 these are distinguished as being 

 petiolate. A leaf may be con- 

 sidered as being formed of two 

 parts, the petiole or leafstalk, 1 5, 

 and the lamina or blade, 16. 

 Leaves are either simple, 14, 



* For a method of obtaining skeleton leaves aj>4 

 plants, see "Enquire Within." 



