ORGANOLOGT. 455 



is closely connected with the presence of the matter of which they consist, 

 so must they be quite dependant upon the chemical processes which furnish 

 this matter. These chemical processes are subject to all the different 

 changes or modifications of acceleration, retardation, &c., which are occa- 

 sioned by variations of temperature, light, pressure of air, and electric 

 tension. In this manner the life of plants is connected through these 

 chemical processes with planetary phenomena, and affected, both mediately 

 and immediately, by planetary changes. Upon these changes depend all 

 the periodical phenomena in the life of plants, the greater part of which 

 are entirely unknown to us, whilst even the more easily comprehended 

 phenomena have been only superficially observed, and this in their 

 relations rather than in their essential forms. 



In the following paragraphs I shall briefly allude to this part of the 

 subject. 



This periodicity displays itself in a double manner : 



1. In certain parts of the plants (as the spore, the pollen-grain, and 

 the embryo), the chemical processes appear almost to stand still, so long 

 as no stimulating external cause excites them. There is, however, by 

 no means an entire cessation of the germinating power of the seed, or it 

 must lie in eternal sleep. This process is constantly carried forward, 

 though sometimes almost imperceptibly. It goes on in different plants at 

 different periods, according as external influences awaken anew the che- 

 mical processes which give to it other directions ; again its activity is 

 extinguished, again to be renewed by external agents, and the result is 

 what we call life. These outward influences only serve to disturb and 

 stimulate the elements of matter, and thus make evident the universal life 

 of nature. 



2. The chemical processes of the entire plant are influenced with great 

 precision by the relations existing between them and the physical changes 

 of the earth and its regions, also the alternation of winter and summer, day 

 and night, and the variation of Aveather produced by them. 



In relation to the first point, we may consider the earth as divided into 

 four regions : 1. The equatorial region, where vegetation never appears to 

 be interrupted, because the heat and moisture are comparatively equalised 

 throughout the year ; 2. The next adjoining region, where the periodical 

 deficiency of moisture retards the chemical process ; 3. The belt lying next, 

 which is of considerable width, and in which the periodical decrease of 

 warmth has the same effect ; and 4. The polar regions, where the small 

 measure, both of warmth and of moisture, almost precludes the possibility 

 of vegetation. 



Of the second, the summer sleep of plants, Martius, in his Flora Bra- 

 siliensis, has given an interesting illustration. We are ourselves, in our 

 latitudes, annual witnesses of the winter sleep of plants, as we observe it 

 at least in the appearances to which it gives rise, though as yet we little 

 understand it. But in this sleep there is only decrease of activity in the 

 chemical processes no actual cessation of them. For though the cold- 

 ness of the temperature may have become such as to cause almost a 

 suspension of these chemical processes, yet they are again stimulated by the 

 action of atmospheric causes ; although, for a short time, the matter of the 

 plant may remain in the same circumstances, the gradual return of the 

 external influences will cause the chemical processes to resume their 

 vigour, and vegetation proceeds in its old course : this may be seen in the 

 careful thawing of frozen parts. 



The alternation of night and day exercises a similar influence, though 



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