Comparative Physiology. 563 



tion, acting like rootlets, and are always most prevalent in the 

 same species, and even in the same individual when trans- 

 planted, in hot dry situations. Plants faded by intense light 

 and heat are refreshed obviously by moisture in the air. 



" In the younger stages of the higher plants, as they approach 

 to maturity, we may trace the same progressive stages of develope- 

 ment in this function as has been done from the lower to the 

 higher plants. The embryo, at its first appearance in the ovule, 

 is nothing but a single cell, like the lowest plants, and gradually 

 absorbs by its whole surface as those do. In the early stages of 

 germination, the first prolonged radicle resembles that of the 

 fungi or mosses ; it is not till the true leaves are developed that 

 the root begins to ramify and produce perfect fibrils, having 

 woody fibre and vessels in the interior, terminated by spongioles. 

 The special structure is thus constantly observed to arise out of 

 one more general; and, even where the special form is most 

 highly developed, the general structure retains, in some degree, 

 the primitive community of function which originally charac- 

 terised it." 



The force by which the sap is propelled upwards, which be- 

 longs more properly to the function of circulation, has by some 

 been described as proceeding from electrical attraction and re- 

 pulsion, producing currents like magnetism. Currents of elec- 

 tricity are supposed to pervade all bodies, and have been said to 

 render the human body susceptible of being mesmerised, or put 

 into a magnetic sleep. Others say that electricity affects all 

 vital action, and will probably be present as partly either cause 

 or effect. The contractile power of the vegetable tissue, which is 

 believed by many the principal agent both in the ascent and 

 descent of the sap, is probably affected by electricity, or elicits 

 it by its action. Professor Thomson ( Veg. Chemis., p. 986.) says, 

 " it is impossible to account for the motion of the sap in plants 

 wholly by any mechanical or chemical principles whatever." The 

 vessels themselves, he says, certainly contract, and many phi- 

 losophers have ascribed the ascent to irritability ; there are not 

 wanting proofs that plants are possessed of it, and Saussure has 

 given a precise view of its mode of action. This power may 

 reside in the vessels along which the sap moves, or in the cells, 

 as DeCandoUe supposes. Miiller (p. 299.) says: " It has been 

 proved by Dutrochet, that the organs which effect the ascent 

 of the sap in plants during the spring are the terminal parts of 

 the root ; that the whole force by which the sap is impelled up- 

 wards is a vis a tergo exerted in the roots. That the attraction 

 of the upper part of the stem is not the cause of ascent was 

 proven by the stem of a vine, cut by Dutrochet at 6 ft. from 

 the ground, continuing to pour forth sap uninterruptedly. That 

 it did not reside in the stem was proven by the flow at the 



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