EXHALATION. 51 



Plicate (plaited), having the surface formed into ridges and channels, by 

 the alternate rising and sinking of the nerves of the leaf; or 



Smooth^ when without wrinkles or ribs ; or 



Villose, or velvety, when covered by soft down or hairs. 



Besides the general form, the character of the margin, and surface of 

 leaves, their position is also described. When upright, and the leaf forms 

 a very acute angle with the stem, it is erect. When they are at right 

 angles with the stems, and parallel with the horizon, they are horizontal. 

 When the apex of the leaf hangs lower than the insertion of the petiole, it 

 is reclined. When the base of the leaf is turned in one direction, and the 

 apex in another, that is, twisted, it is oblique. 



Radical leaves are those which grow very near to the root. 



When leaves arise one after the other from opposite sides of the stem, 

 they are alternate ; but when they arise, on the same line, from opposite 

 sides of the stem, they are opposite. 



When they grow in a circle round a stem, they are verticillate (whorled) 

 or stellate. 



EXHALATION. 



30. When treating of absorption, we saw that vascular plants 

 pump up, by their roots, a considerable quantity of water, hold- 

 ing different matters in solution, and that this liquid rises through 

 the stem to reach the leaves. But all the water thus absorbed 

 does not remain in the interior of the plant, and a great part is 

 dissipated in the form of vapour. To satisfy ourselves on this 

 point, it is only necessary to place in a perfectly dry glass jar, 

 the leafy stem of a vegetating plant, and expose the whole to the 

 sun ; we soon discover little drops which arrange themselves on 

 the parietes of the jar. By weighing plants immediately after 

 they have been watered, and weighing them again some time 

 afterwards, we obtain proof of this loss, and we may exactly 

 estimate the quantity of water exhaled ; it was found, by an 

 experiment of this kind, that a cabbage lost by evaporation nine- 

 teen ounces of water a day, and a helianthus (from the Greek, 

 elios, the sun, and anthos, flower) or sunflower loses even a more 

 considerable quantity in form of vapour. 



31. A small part of the water thus expelled, evaporates through 

 the tissue which constitutes the surface of all parts of the plant, 

 as well after death as during life ; and it is for this rtnson that 

 the stem, fruit, tubercles, and flowers terminate their existence 

 by drying, when the place in which they may be is not very 

 damp. But the greatest quantity of water is expelled through 

 the leaves of the living plant, and this exhalation only takes 

 place, while the plant is alive, and when the influence of light 



30. What becomes of the water absorbed by the roots ? How is it ascer 

 tained that plants exhale water in form of vapour? What quantity of 

 water does a cabbage exhale ? 



31. What parts of plants are seats of exhalation ? When does exhalation 

 tah e place ? What influences exhalation ? What description of plants* 

 exhale least? 



