76 ANATOMY AND PHYSIOLOGY OF 



is connected with the strong evaporation; this not only causes the 

 sap within them to become more concentrated and thus more 

 capable of attracting to itself, through endosmose, the sap con- 

 tained in the cells of the stem (a property which the sap contained 

 iB the leaves acquires the more since its organic, especially gummy, 

 compounds are formed out of inorganic substances), but, as Liebig 

 has shewn, the evaporation from the superficial cells causes the 

 flow of sap towards them by itself, and independently of the en- 

 do&mose they exert. The ascent of the sap through the cells of 

 the stem to the leaves is indeed explicable in this way; but in 

 what way does the activity of the leaves cause fluids in which 

 the open ends of the vessels of a cut stem dip, to be absorbed by 

 the vessels and conveyed upwards in them? That endosmose has 

 no share in this is self-evident, for all the conditions to induce it are 

 wanting. Equally insufficient is the explanation given by L. W. 

 Th. Bi&choff ("De vera vasor. spiral, natur. etfunct/' 62). Accord- 

 ing to his view, the air contained in the vessels is absorbed by the 

 sap of the cells in the different parts of the plant, and used for 

 the chemical transformation of their contents, consequently a fluid 

 which is in contact with the open mouths of vessels must be 

 driven into them by the pressure of the atmosphere. Were this 

 correct, a shoot of which the end was cut off and its vessels there- 

 by opened at their upper extremities, or a tree from which many 

 branches have been cut off, so that the vessels are in communica- 

 tion with the external air in many places, could not absorb fluid 

 into these vessels. 



But in the ascent of the sap there occurs another phenomenon, 

 which cannot be explained by the endosmose exercised by the 

 cells; namely, the endeavour of the plant to carry up the sap 

 more especially in a perpendicular direction. It is a well-known 

 phenomenon that the bud which stands upon the end of a shoot 

 receives the most sap ; that it grows out into a stronger shoot than 

 those situated lower down; that of two shoots of which one is 

 brought into a vertical position, the other bent sideways or down- 

 wards, the growth of the former is favoured, and that of the other 

 interfered with. The endosmotic force of its cells cannot be altered 

 by this change of position, and yet the strength of the current of 

 sap going to the shoot is altered. 



All these explanations of the movement of the sap bear reference 

 only to its ascent, not one of them applies at all to the descent 

 of the elaborated nutrient sap. If the bark and the cambium 

 layer attract the nutrient matter from the leaves because their 

 cells contain a more concentrated sap than the cells of the leaves, 

 it is not evident why they cannot draw the sap directly from the 

 root and the wood, instead of by the long circuit through the 

 leaves, and why the bark is wholly incapable of carrying sap up- 

 wards. 



Gathering all these circumstances together it seems to me to 



