68 



Elementary Plant Physiology. 



Fig. 46. — Transverse section tiirougll stoma of Iris 

 florentina. After Strasburger. 



203.) Among the 

 lower forms of plants, 

 openings through ex- 

 ternal layers of the 

 body are not always 

 under the control of 

 the plant. If the 

 student is not already 

 familiar with the thal- 

 lus of Marchantia, 

 cross-sections should 

 be made, to obtain a 

 view of the structure of the openings. 



62. Nature of transpiration. — The mineral salts ab- 

 sorbed by the roots are used in 

 all parts of the body, but in 

 greatest quantity in the leaves. 

 These salts are absorbed in the 

 proportion of about one part to 

 ten thousand parts of water. 

 Transpiration may be regarded 

 as a device for conveying salts 

 to the leaves and upper parts of 

 the plant. As a result of this 

 process, a constant stream of 

 water sets from the roots to the 

 leaves, carrying mineral sub- 

 stances at a much faster rate 

 than might be done by diffu- 

 sion, and, having reached the 

 extremities of the body, a large 

 part of the water evaporates, and Fig.47.— Surface, ttiree-fourths pro- 

 leaves the salts behind. An «''=■ ^""^ p™'"^ ^'7^ of stomata of 



Schizxa pusilla. After Bntton and 



enormous amount of water is Taylor. 



