MI.M..IK XXVI.] CAl'lLLAHY ATTKACT1ON, ETC. 



comes ? Because it controls the amount of carbonic acid 

 reduced, and, therefore, the amount of elaborated sap 

 formed. Why is it that the upward flow diminishes 

 when changes are befalling the leaves, and why does it 

 stop in the winter ? Because the mucilaginous solution 

 made by light diminishes in quantity, or ceases to be 

 formed altogether. 



There are, therefore, two sources of force in a flower- 

 ing plant the spongiole and the leaf; and they derive 

 their power from ordinary physical principles. What- 

 ever has been said respecting the movements of sap in 

 exogenous plants applies also to the case of endogenous, 

 and indeed to flowerless plants too. 



It has been clearly established by the researches of 

 comparative anatomists that the presence of a circula- 

 tory mechanism is determined by the centralization of 

 the nutritive and respiratory apparatus. In exogenous 

 and endogenous plants, from the circumstance that liquid 

 and solid materials are introduced at distant points, chan- 

 nels of communication from one to the other, and indeed 

 to every part, are required, and hence the introduction 

 of a circulatory apparatus. In the lower tribes of vege- 

 table life, where the separation of function does not ex- 

 ist, the circulatory mechanism is correspondingly absent. 

 Sea-weeds absorb on their whole surface, and nutrition 

 is directly carried forward at the points of reception. 

 In lichens there is the first appearance of a transfusory 

 mechanism, arising from the circumstance that on those 

 parts which are shaded from the light, absorption most 

 rapidly takes place : here, probably, however, the chan- 

 nels of movement are the interspaces between the cells, 

 and the cause simple capillary attraction. In mush- 

 rooms there is a closer approximation to the mechanism 

 more fully developed in the higher plants, for in them 

 the rootlets absorb nutrient matter from the soil, from 



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