66 REVIEWS. 



layers of wood are deposited, more fruit is perfected, tliis 

 ripens sooner, etc. The deposition of starch which occurs in 

 the cells of the medullary rays in autumn, goes to prove that 

 the portion of assimilated sap which is not used for nutrition 

 on the way to the root, runs back to the wood through these 

 horizontal medullary rays ; and thus the sap describes a kind of 

 circle, not, indeed, in determinate vessels, but in a definite path 

 leading through the different parts of the plant. It is difficult 

 to see how in recent times the results of these experiments 

 could have been so questioned, and the existence of the de- 

 scending current in the bark denied. Certainly it is no im- 

 provement on the theory cast aside when the increased growth 

 above the annular wound is explained by the artificial inter- 

 ruption of the upward current of crude sap, in consequence of 

 which the sap contained in the upper part of the plant must soon 

 become greatly concentrated and potential for development 

 (Schleiden, ' Grundziige,' 2d ed., ii. 513). When we can 

 succeed in fattening an animal by depriving it of a portion of 

 its accustomed food, this explanation may be received as satis- 

 factory " (p. 70, 71). After some excellent points of criti- 

 cism, Mohl concludes that the discovery of endosmose has not 

 fully solved the problem of the movement of the sap in plants, 

 although in all probability it does play an important and j^er- 

 haps the principal part in its absorption and conve3^ance 

 (p. 77). To the question, whether plants live on inorganic 

 food alone or take in also organic matters, Mohl gives a sen- 

 sible answer, rejecting the extreme view of Liebig, while still 

 fully recognizing the great office and result of vegetation 

 (p. 78). According to Mohl, however, it is proved that plants 

 do not absorb the carbonic acid dissolved in water with the 

 latter by means of their roots (p. 81) ; but this seems hardly 

 reconcilable with several facts stated on the next page, from 

 which it is justly concluded that carbonic acid is carried 

 up with the ascending sap into the leaves. From the fact 

 that plants perish so soon in air deprived of all oxygen gas, 

 that sensitive leaves lose their irritability under such circum- 

 stances, etc., Mohl concludes, apparently with good reason, 

 that the absorption of oxygen and the exhalation of carbonic 



