12 FUNCTIONS OF THE STEM 



1. The roots. The roots mechanically absorb or drink in, 

 from the soil, the various substances which the plant can derive 

 only from the soil. In doing so, they refuse to take up all 

 substances, even when they are readily soluble in water, indis- 

 criminately or in equal proportions. They appear to exercise 

 a kind of selecting power, inasmuch as, while some substances 

 are taken in readily and abundantly, others enter with diffi- 

 culty, or in small comparative quantity. In this respect, the 

 roots of different plants also exhibit diversities. It is probable 

 that the structure and substance of the pores or cells, through 

 which the food passes into the extremities of the roots, vary 

 with the genus or species of plant, and that such variations 

 have an influence upon the proportions in which different sub- 

 stances are absorbed by them. 



Upon the food, after it has entered, the roots exercise a che- 

 mical action, at least such an action is exercised upon it as it 

 ascends towards the stem. Scarcely has a substance been ab- 

 sorbed than it undergoes a chemical change more or less sen- 

 sible. Coloured substances, such as madder, are seen to become 

 colourless ; but our knowledge regarding these chemical changes 

 is general only, and we can as yet do little more than guess at 

 their nature, from the substances which we afterwards find in 

 the sap. 



2. The stem transmits the fluids and food absorbed by the 

 roots upwards to the leaves. This is its mechanical action. As 

 it ascends, the sap changes. The substances it holds in solution 

 are decomposed or compounded, and thus converted into new 

 chemical combinations. It is in reference to these changes that 

 the stem appears to perform a chemical function. But here 

 also our knowledge is limited. We know that certain solid 

 substances, such as wood and starch, are deposited and fixed ; 

 certain others, such as sugar, formed in the sap ; and certain 

 others, again, given off in the state of gas or vapour from the 

 leaves ; but we cannot tell at what point these changes take 

 place, by what immediate agencies they are effected, nor through 

 what successive steps of change the elementary bodies proceed 

 in their progress towards the results which we discover to have 

 been at last produced. 



