54 



METABOLISM 



another, viz. the wound inflicted on the plant. Very frequently the bleeding 

 commences at once after the incision has been made, and then it is mani- 

 fest that the vessels were already charged with water, and that the incision 

 merely provided an opening for the exit of the fluid ; but in other cases it is not 

 so, for the exudation may not begin to take place until some time after the 

 wound has been made ; in such cases the actual wounding itself must have 

 been the cause of the exudation. Until quite recently it was customary to 

 attribute the exudation of sap containing large quantities of sugar from in- 

 cisions made into the young axes of inflorescence of certain palms [Cocos nucifera, 

 Arenga saccharifera, &c.) to the influence of root-pressure. According to 

 MoLiscH (1898), however, no such root-pressure exists in these palms, and no 

 sap exudes either from the stumps of the trees when felled, nor from auger 

 holes bored in their stems ; nor is there any excretion from the inflores- 

 cence itself, if only a simple incision be made in it. In Cocos nucifera bleed- 

 ing first begins to take place after the apices of the inflorescence have been 

 cut for several days in succession, and in Arenga a much greater effect is 

 produced if, during the four or five weeks previous to flowering, repeated 

 bruises be inflicted on the base of the boll by blows from a wooden hammer 

 (a practice followed by the Malays) ; then, when the inflorescence is cut off, 

 secretion begins at once. Nor are these altogether isolated observations. 

 BoHM (1892) was the first to observe the high pressures mentioned above 

 (more than eight atmospheres), which might be obtained from our native 

 trees by means of manometers fixed for a long period in auger holes bored 

 in their stems ; and Molisch (1902) has drawn the conclusion that these 

 pressures have nothing to do with root-pressure, seeing that the trees at 

 the time of the experiment were covered with leaves, and when ex- 

 perimented upon with fresh auger holes gave, as indeed might have been 

 expected, no positive pressure at all, sometimes even a negative one. Here 

 also the secretion was, in the first instance, traceable to the actual wound 

 itself and produced gradually and in its immediate vicinity. It originated, 

 in all probability, from cells which arise or are stimulated to further growth in 

 consequ< nee of the wounding. At the same time various infiltration products 

 enter the lumina of the vessels in the neighbourhood of the wound and render 

 that part of the wood quite impermeable to water. It is quite evident, there- 

 fore, that a marked pressure may be produced purely locally, although a scarcity 

 of water prevails in the immediate vicinity. Molisch speaks of such cases as 

 local pressures, and it is very probable that local bleeding may take place not 

 merely in palms and other trees above mentioned, but generally in all cases 

 of amputated branches and leaves, where the existence of bleeding has been 

 established (Pitra, 1878), whilst what may be termed normal bleeding is associ- 

 ated, perhaps, with the root only. 



That bleeding, i. e. the unilateral excretion of water from parenchyma 

 into vessels, is an osmotic phenomenon, has been generally accepted since the 

 time of DuTROCHET. In order to obtain an explanation of this unilateral 

 excretion, however, we must now study examples of plants which exude sap 

 of very weak concentration, and we may assume that the cells concerned are 

 lined by protoplasm which is completely impermeable to the substances dis- 

 solved in the vacuole. Then comes the question, how can a unilateral expres- 

 sion of water take place from such a turgescent cell ? In an ordinary cell the 

 all-round endosmosis of water is balanced by the exosmosis caused by the 

 pressure from within ; as in one place more water will exude than enter, so in 

 another place more water wifl flow in than flow out. In order to explain the 

 varying behaviour of the different sides of the cell it has been the custom 

 hitherto to ascribe it to the different quahties of the plasmatic membrane, 

 since it was believed that the degree of osmotic pressure was dependent 

 on the character of the plasmatic membrane. If one half of the cell possesses 



