560 Comparative Physiology. 



texture, and of a brown colour, as if pervaded witli the tannin. 

 It probably acts by its astringent property causing the pores of 

 the spongiole to contract. 



Dr. Madden thinks it possible that plants may have an in- 

 stinctive relish for that kind of food which contains the requi- 

 sites needed for their present wants in greatest perfection. It 

 does not appear, however, that plants have any thing like in- 

 stinct in the power of selecting by the spongioles ; this is pro- 

 bably confined to the assimilating organs. It has been said that 

 the power of selecting is proved by such plants as peas having 

 little silex in their composition, while others, such as grain crops 

 (grasses), are found to abound in silex in the stems. The one plant 

 abounding in one substance in which the others are deficient, 

 thouffh P-rowing beside each other, and having the same food 

 supplied to the roots, is thought to be a j)roof of a selecting 

 power in the spongioles. This power, however, more probably 

 resides in the assimilating organs of the grasses possessing a 

 power of appropriating silicates, which those of the pea do not 

 j)ossess. In the experiments before noticed of Yogel, it was 

 found that plants in general absorb rapidly so much of saline 

 substances as to destroy them, even of such as nitrate of potash, 

 which is well known to benefit plants in small quantities. 

 Saussure found solutions of sulphate of copper and siigar to 

 kill plants more rapidly than most other substances, which were 

 the two of which the greatest quantity was absorbed.* 



It is evident, therefore, that, as small quantities of the delete- 

 rious substances of Vogel are to be found in many soils natu- 

 rally, and as even the most nourishing, as sugar, were found 

 to be hurtful in excess, plants must possess a power of ex- 

 creting deleterious substances, which would otherwise collect and 

 destroy. The fluid in the soil may be absorbed by the pea and 

 the grass of the same quality unaltered, only the pea will have 

 to reject and excrete the silicates that are not needed for its 

 stalk ; and so with any plant for any substances not needed. 

 Some few plants may possess a particular power of rejecting 

 some substances, as the chara found by Vogel not to absorb 

 sulphate of copper, probably from the form of its pores. But 

 few instances are on record, however, of such powers being 

 possessed ; on the contrary, it seems generally allowed, that all 

 soluble substances, deleterious or not, are absorbed indiscrimi- 

 nately. It follows, therefore, as some deleterious substances 

 may very soon collect in such quantities as to be hurtful, and as 



* He also found that, when the roots were cut, all substances were ab- 

 sorbed with equal rapidity, showing that the form and size of the invisible 

 pores had regulated the quantities before they were cut. Boucherie, it is 

 said, in the absorption of saline substances by trees, found the cut ends of 

 certain trees to reject some substances which other trees took up. 



