RECENT HYPOTHESES. 225 



when the period of growth is over, this process is reversed, 

 more sap being given off by the roots than is taken up by 

 them." Now, this is pure assumption, there being no proof 

 that the sap of trees escapes from the roots in autumn. In 

 fact, it appears that the wood of trees contains as much sap in 

 winter, when at rest, as in the period of most active growth. 



Again, Dr. Pettigrew remarks : " It is difficult to understand 

 how excess of moisture in the ground can be drawn up into 

 the plant and exhaled by the leaves at one period, and excess 

 of moisture in the atmosphere seized by the plant, and dis- 

 charged by the roots at another. The explanation, however, 

 is obvious, if Ave call to our aid the forces of endosmose and 

 exosmose. The tree is always full of tenacious, dense saps, 

 and it is a matter of indifference whether a thinner watery fluid 

 be presented to its roots or its leaves ; if the thinner fluid be 

 presented to its roots, then the endosmotic or principal cur- 

 rent sets rapidly in an upward direction ; if, on the other hand, 

 the thinner fluid be presented to its leaves, the endosmotic or 

 principal current sets rapidly in a downward direction." 



This explanation is not only false, but superfluous, since no 

 such circulation can be shown to exist, but is an excellent sam- 

 ple of the common mode of dealing with this obscure subject. 

 Instead of seeking to discover the exact facts concerning the 

 composition and movements of the sap in all parts of the 

 plant, a display of book-knowledge is made by quoting from 

 numerous writers of some repute, such statements as seem to 

 corroborate the hypotheses of the author. The assumed phe- 

 nomena of the circulation are then accounted for in an appar- 

 ently scientific manner by ingenious allusions to osmose, cap- 

 illarity, and other physical forces, the surprising possibilities 

 of which are duly recounted. 



Dr. Pettigrew further observes, that "Herbert Spencer be- 

 lieves that the upward and downward circulation of crude and 

 elaborated saps takes place in a single system of vessels or 

 vertical tubes." To explain this extraordinary assumption, 

 Mr. Spencer states that "the vessels of the branches termi- 

 nate in club-shaped expansions in the leaves, which expan- 

 sions act as absorbent organs, and may be compared to the 

 spongioles of the root. If, therefore, the spongioles of the 

 root send up the crude sap, it is not difficult to understand 

 29 



