SUPPLEMENT 15 



even more probable in the case of the more sacchariferous sap of Acer, Agave, 

 and Palmaceae. It is quite out of the question, however, to regard bleeding- 

 pressure in general as osmotic pressure in this sense, for we find no relation 

 existing between the height of the pressure of the sap and its concentration ; 

 in the vine more especially a feeble concentration of the sap occurs along with 

 high pressure. WIELER has instituted experiments specially with the view 

 of settling this point ; he permitted osmotically active solutions to be absorbed 

 by the vessels of the root-stump, but failed to induce bleeding thereby. 



Further, one has difficulty in assuming that such large amounts of sugar 

 as are obtained from palms and Agave can arise from the cell-walls ; they 

 must be produced from their interiors. They must therefore pass through 

 the plasma, and that layer cannot therefore be as impermeable as we have 

 hitherto regarded it. As soon, however, as we assume a unilateral permeability 

 of the plasma the conditions of unilateral expression of water are again 

 fulfilled, because in that way certainly a permanent difference in the con- 

 centration of the sap on different sides of the cell is established. If also the 

 qualitative difference of the plasmatic membranes in different regions of the 

 cell consists in its impermeability on the one hand and partial permeability 

 on the other, a unilateral exudation of fluid may then certainly result. One 

 should note, however, that it is never pure water but always cell-sap that 

 filters out, though that sap may certainly be very dilute. 



At present we are not in a position to advance decisive reasons for the 

 acceptance of one or the other of the explanations offered of the unilateral 

 exudation of fluid from the cell. Still, after weighing probabilities, one would 

 rather incline to the view that the condition essential to bleeding lies in a differ- 

 ence of concentration of sap on opposite sides of the cell, induced and main- 

 tained often (possibly in the case of dilute sap) by an expenditure of energy 

 on the part of the cell, but also often (in the case of concentrated sap) by the 

 unilateral permeability of the protoplasm. 



1. 25, delete a secretion 



1. 46, for the significance to be attached to read its significance in this case 

 to lie in 



60, 1. 2, for save read besides 

 1. 5, for e. g. read even 



Bibliography, for HALES. 1748, &c. read HALES. 1731-3. Vegetable 

 Staticks. London. 



61, 11. 16-23, for Indeed the older physiologists . . . (1891, p. 938), read 

 This fact is shown more clearly by an experiment performed by STRASBURGER 

 on the copper beech (1891, p. 938). 



I. 34, for A glance at read But the estimate is certainly not an exact one, 

 for a glance at 



1-35. f or shows clearly that read shows more clearly than an experiment 

 that 



62, 11. 19-20, for and it will . . . that this plant read and thus only can we 

 explain how, as was shown in the last lecture, this plant 



II. 24-8, for If a sharp line ... be given read If a sharp line of demarca- 

 tion existed between the heart and sap wood, and if we could assume on the 

 one hand that the whole of the sap wood was of equal value for conductive 

 purposes, and that the whole of the heart wood was valueless, still only approxi- 

 mate estimates of the area of the wood actually concerned in the transport of 

 water could be arrived at. 



63, 11. 9-12, for in the plant, since . . . raised in the plant read in the plant. 

 In all the numerous explanations hitherto offered no attempt has been made 



