314 STATE POMOLOGICAL SOCIETY. 



plant. In autumn, 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 difiBcult 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 discharged by the roots at another. The explanation, however, is 

 obvious, if we 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 current sets 

 rapidly in an upward direction ; if, on the other hand, the thinner fluid be 

 presented to the 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 sample of the common mode of deal- 

 ing 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 phenomena of the circulation are then accounted for 

 in an apparently scientific manner by ingenious allusions to osmose, capillarity, 

 and other physical forces, the surprising possibilities of which are duly re- 

 counted. 



Dr. Pettigrew further observes, that " Herbert Spencer believes 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 terminate in 

 club-shaped expansions in the leaves, which expansions act as absorbent organs, 

 and may be compared to the spongioles of the root. If, therefore, the spongi- 

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

 these spongioles of the leaf send down the elaborated sap, one channel sufficing 

 for the transit of both." This hypothesis concerning the circulation of sap is 

 accepted only by its inventor, and is directly opposed to most of the facts of 

 plant grow til. 



Finally, Mr. Pettigrew has conceived a system of syphons by the aid of which 

 he is able to account to his entire satisfaction for all he knows concerning the 

 circulation of sap. He says: '-'The vessels which convey the sap, as is well 

 known, are arranged in more or less parallel vertical lines. If the vessels are 

 united to each other by a capillary plexus, or, what is equivalent thereto, in 

 the leaves and roots, they are at once, as has been shown, converted into syphon 

 tubes, one set bending upon itself in the leaves, the other set bending upon 

 itself in the roots. As, however, a certain portion of the syphon tubes which 

 bend upon themselves in the roots are porous and virtually open towards the 

 leaves, while a certain portion of the syphon tubes which bend upon them- 

 selves in the leaves are porous and virtually open towards the roots, 

 it follows that the contents of the syphon tubes may be made to move by an 

 increase or decrease of moisture, heat, etc., either from above or from below* 

 In spring the vessels may be said to consist of one set, because at this period 



