282 DISEASE OF PLANTS. 



formed seeds or reproductive particles, which -will 

 insure a fresh growth of the same kind of plant next 

 year; after this, vitality gradually declines in energy, 

 the formation of fresh organic matter ceases, and 

 that already formed begins to decompose. 



742. In following out the chemical changes which 

 occur during the life of a plant, we find all through- 

 out, that there is a constant struggle going on be- 

 tween vital force and the ordinary chemical affinities 

 of the elements of vegetable matter. The influence 

 of vital energy tends to form complex combinations, 

 whilst the natural affinities of the elements tend to 

 form simple ones. So long as the former is the more 

 powerful, the plant grows and flourishes ; as soon, 

 however, as the latter gets the upper hand, the plant 

 withers and fades — disease, and perhaps death, fol- 

 low. 



743. This kind of struggle is well seen in the an- 

 nual fall of the leaf. During the whole of the sum- 

 mer, whilst the vital energy is great, lignin, starch, 

 sugar, and other similar compounds, are formed in 

 the leaves, and through their agency; towards the 

 close of the year, when they have fulfilled the ob- 

 jects for which they were formed, these eff'ects cease, 

 the vital power diminishes, and ordinary chemical 

 agency gets the ascendant. The first effect of this 

 is oxidation ; the leaves begin to absorb oxygen, their 

 green coloring matter oxidizes and becomes yellow, 

 the leaves wither, fade, die, and fall off". But even 

 then chemical power continues to act on them ; they 



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