394 PLANT RESPONSE 



compression, will produce a bulging or over-turgid ity, and a 

 manometric tube inserted laterally will indicate a positive 

 pressure. Or if, under these circumstances, we make a cut in 

 the pipe, we shall observe exudation under pressure. Now, 

 this corresponds to the exudation pressure, causing so-called 

 ' bleeding,' which we find on making incisions in plants in 

 spring time, when the loss of water by transpiration is feeble, 

 the buds being still unfolded ; or even in summer, if trans- 

 piration be by any means prevented. 



In connection with this exudation of sap, we have to 

 remember that the whole question is one of income and ex- 

 penditure. Generally speaking, the loss by transpiration is 

 less at the end of winter or the beginning of spring than in 

 other seasons. Thus the wild date palm, or P/iccnix syhestris, 

 yields considerable quantities of sugary sap from incisions in 

 the stem, at the end of winter or the beginning of spring. In 

 the case of the Palmyra palm (Borassus fiabelliformis) of 

 Bengal, however, the increase of cellular activity in summer 

 more than compensates for the loss by transpiration, and sugary 

 juice is collected in the height of summer at the top of the 

 tree by incisions in the peduncle. The pressure exerted by 

 the sap may be gathered from the fact that these trees are 

 often more than a hundred feet high. 



(2) Negative pressure. — We may next suppose that in 

 the chain of pumps, those at the upper end are the most active, 

 and the aperture of escape wide open, the removal of water 

 being further aided by evaporation. The loss of water being 

 thus greater than the supply, it is clear that there will be a 

 negative pressure in the pipe, and the mercury in a testing 

 lateral manometer will be sucked in. This corresponds to 

 the negative pressure exhibited by an actively transpiring 

 plant. In such a case it will be noted that the activity of the 

 rhythmic cells, which is the fundamental cause of the ascent 

 of sap, is further aided by the evaporation from the leaves. 

 Concentration of cell-sap also, and the osmotic action 

 thereby produced, may then constitute an additional auxiliary 

 factor. 



