PHYTO-PHYSIOLOGY. 2 .VJ 



B. Preliminary Events. 



a WROUGHT BY EXTERNAL INFLUENCE. 



1371. The influence of the elements produces differ- 

 ent phenomena in the plant. I have felt constantly 

 more inclined to consider, that not merely the descent 

 of the root, but even the ascent of the stalk was simply 

 to be viewed as a mechanical event, or one forsooth 

 effected by gravity. The roots obey under all circum- 

 stances the gravity and would grow as far as the centre 

 of the earth, were they to meet with no impediment ; 

 and there they would follow the revolution of the earth, 

 and consequently become spirally convoluted upon them- 

 selves. It is almost beyond doubt that the water, which 

 sinks downwards and, as it were like that in stalactites, 

 invariably rigidifies or hardens at the radical capillaries, 

 is heavier in the root. The cause of this greater weight 

 depends upon the mucus not being decomposed. 



1372. The straight ascent of the stalk also depends 

 upon nothing else than gravity. The upper drops of 

 mucus become lighter by means of greater heat and by 

 decomposition in light and air, and they are therefore 

 compressed by the heavier in the upward direction. It 

 is always such a small drop upon the summit, which 

 hardens into its uppermost cell. The stalk, therefore, 

 grows upwards through the same forces and in the same 

 manner, as the air- bubbles ascend in a glass of beer. 

 The cause of their becoming lighter resides certainly in 

 the vital process, which nevertheless effects in this respect 

 nothing else than the extension or increase; but yet 

 the cause of the ascent is naught else than the gravity. 



1373. The light likewise acts upon the direction of 

 plants and especially that of their leaves ; not simply 

 from its promoting growth by elevation of temperature 

 and by decomposition, but obviously in a mechanical 

 manner also ; for not only do the branches of plants in a 

 green-house grow towards the window, but most leaves 

 turn themselves the whole day in obedience to the course 



