THE SAP OF PLANTS. 81 



be no reasonable doubt that it is also dispersed througli the whole 

 system, by means of some permeable quality of the membranes of the 

 cellular tissue, which is invisible to our eyes, even aided by tlie most 

 powerful glasses. It has also been suggested that the sap finds its way 

 upwards, downwards, and latterly through the intercellular passages, 

 which exist at tlie points of union of every individual elementary organ. 

 That such a channel of communicating the sap is employed by nature 

 to a certain extent J do not doubt, especially in tliose plants in whicli 

 the intercellular passages are large ; but whether this be an universal 

 law, or has only a partial operation, is quite unknown, and is not, 

 perhaps, susceptible of absolute proof. 



" The accumulation of sap in plants appears to be attended with very 

 beneficial consequences, and to be deserving of the especial attention of 

 gardeners. It is well known how weak and imperfect is the inflo- 

 rescence of the turnip tribe, forced to flower before their fleshy root is 

 formed, and how vigorous it is after that reservoir of accumulated sap 

 is completed. Mr. Knight, in a valuable paper on this subject, remarks 

 that the fruit of melons, wliich sets upon the plant when very young, 

 uniformly falls off; while, if not allowed to set until the stem is well 

 formed, and much sap accumulated for its support, it swells rapidly and 

 ripens well. In like manner, if a tree is by any circumstance prevented 

 bearing its crop one year, the sap that would iiavebeen expended accu- 

 mulates, and powerfully contributes to the abundance and jierfection of 

 the fruit of the succeeding year. 



" The course of the motion of the sap is a subject which has long 

 excited great curiosity, and has given rise to numberless conjectures. 

 It was for a long time believed that there was a sort of circulation of 

 tlie sap of plants to and from a certain point, analogous to that of the 

 blood of animals ; but this was disproved by Hales, and is not now 

 believed. This excellent observer thought that the motion of the sap 

 (tlie rapidity of which he had found to be greatly influenced by the 

 weather) depended upon the contraction and expansion of the air, which 

 exists in great quantities in the interior of plants. Others have ascribed 

 the motion to capillary attraction, and Du Petit Thouars suggests that 

 it arises thus : — ' In the spring, as soon as vegetation commences, the 

 extremities of the branches and buds begin to open. The instant this 

 happens, a certain quantity of sap is attracted out of the circumjacent 

 tissue for the supply of these buds ; the tissue which is thus emptied of 

 its sap is filled constantly by that beneath or about it ; this is in its turn 

 replenished by the next ; and thus the mass of fluid is set in motion, 

 from the extremities of the branches down to the roots.' Du Petit 

 Thenars is, therefore, of opinion that the expansion of the leaves, &c., 

 is not the effect of the motion of the sap, but, on the contrary, the 

 cause of it, and that the sap begins to move at the extremities of the 

 branches before it stirs at the roots. Tliat this is really the fact is well 

 known to foresters and all persons accustomed to the felling or exami- 

 nation of timber-trees in the sprino:." 



