56 METABOLISM 



Then again, it is very difficult to believe that an amount of sugar as great 

 as that which may be obtained from palms, Agave, and other plants, comes 

 from the cell-walls ; obviously it must have been formed in the interior of the 

 cell, and, inasmuch as it passes through the protoplasm in order to reach the 

 exterior, the plasma cannot be so impermeable as we have hitherto been 

 led to believe. So soon as we postulate a unilateral permeability for the proto- 

 plasm, however, the conditions necessary for unilateral excretion of sap are 

 fulfilled, because in this way a permanent difference in the concentration of 

 cell-sap on the different sides of the cell is established. If the qualitative difference 

 in the plasmatic membrane at different situations in the cell consists in its 

 being impermeable on one side, on the other partially so, unilateral excretion of 

 water becomes possible ; but the fluid that filters through is always cell-sap, 

 however dilute it may be, and never pure water. 



We are not in a position at present to advance conclusive evidence in favour 

 of one rather than another of the explanations which have been put forward 

 to account for bleeding ; but after due consideration we are inclined to favour 

 the view that bleeding is to be accounted for by differences in the concentration 

 of cell-sap on two sides of the cell, differences which are often produced and 

 maintained by expenditure of energy on the part of the cell itself, often also by 

 unilateral permeability of the protoplasm. 



Bleeding, so far as we have considered it, though of the greatest impor- 

 tance to the plant physiologist, cannot be regarded in any other light than as 

 an injurious and even pathological process to the plant itself. No matter 

 whether it be pure water or a concentrated sugar solution which escapes from 

 the wound, the plant is always suffering a loss of material without receiving 

 any compensation. In the uninjured plant we find that an excretion of 

 water takes place under pressure into the vascular strand. It is true we are, 

 as a rule, unable to prove this fact directly, since the exudation of water is 

 manifested only after the infliction of the wound ; but the fact that at certain 

 times and in certain plants excretion of water takes place immediately upon 

 the cutting off of a branch, proves indubitably that a bleeding pressure exists 

 in the plant, although no incision has been made. T. Hartig (1853, 1862) has 

 already observed that in spring, before the unfolding of the leaves, sap exudes 

 from the buds of the hornbeam and other trees, although no lesions are visible. 

 Strasburger (1891, p. 840) has also more recently demonstrated that this 

 extravasation of drops is a result of bleeding pressure, and that drops exude 

 from the apices of the leaves of the previous year, whose cuticle has been burst 

 open. This feature is by no means an annual one, nor is it observable in all 

 hornbeams, hence we must conclude that an especially high pressure is neces- 

 sary, not only, in the first place, to force the water up to the apices of the 

 branches, but also, in the second place, to overcome the opposition which the 

 leaf apices present to the outflow. 



What is in arboreal plants the exception is the rule in many herbaceous 

 plants ; for under favourable conditions, especially excessive dampness of the 

 soil and reduced transpiration, conditions found during the night, water is 

 forced into the whole vascular system of such plants, mainly through the activity 

 of the root, with such force that an exudation of drops takes place wherever 

 opposition to filtration is reduced. A noteworthy example of this phenomenon 

 is furnished by the leaves of Colocasia antiquorum, and other Aroidaceae, e. g. 

 Reniusatia vivipara show it also. The drops in these cases exude exclusively 

 from the apices of the leaves and succeed each other very rapidly. In Colo- 

 casia, DucHARTRE (1859) counted ten to fifteen, and, in extreme cases, as many 

 as thirty, drops falling from the leaf-apex every minute. Each drop, more- 

 over, was formed from the coalescence of five to six smaller drops expressed 

 simultaneously from the leaf, so that the leaf gave off a maximum of 180 drops 

 per minute, or three per second, and the total quantity of fluid which could be 



