MINERAL MATTERS IX FALLEN LEAVES. 17 



while the bark of the branches is, just at that time, often 

 actually overflowing with sap (H. Mohl). In accordance 

 with this fact, the analysis of the ash of the leaves shows 

 that the amount of alkali and phosphoric acid in them 

 decreases immediately before the fall ; the fallen leaves 

 contain such trifling quantities of these constituents, in 

 comparison to their mass, that it is difficult to account 

 for the injmious consequences arising fi^om the raking 

 up and removal of the fallen leaves in woods. (See 

 Appendix A.) 



A similar reflux of the assimilative products appears to 

 take place in the grasses ; when from the intense heat of 

 summer the leaves begin to decay, chemical analysis 

 reveals in the yellow leaves scarcely any traces of nitrogen, 

 phosphates, and alkalis ; and, indeed, animals instinctively 

 tiu-n from all kinds of fallen leaves, and refuse to feed on 

 them. 



In annuals and biennials the organic action residts in 

 the production of fruit and seed, after which the activity 

 of the root comes to an end ; in perennials, the production 

 of seed is rather an accidental condition of their permanent 

 existence. 



The biennial can bestow more time than the annual in 

 gathering the material necessary for the production of 

 seed and fruit, which closes the period of its existence ; 

 but the time in which this takes place depends upon the 

 state of the weather and the nature of the soil. 



The annual is uniformly developed in all its parts ; the 

 food daily taken up is expended in increasing the over- 

 ground and underground organs, which meanwhile take 

 up a larger amount of food in proportion to the increase 

 of their absorbent surface. With tlie growth of the 

 plant, the conditions of increase inherent in the plant 



c 



