260 HOW CEOPS GROW. 



trees ajspears to result from -wounds or disease, and the 

 matters whicli in the experiments of Gyde and others 

 were observed to he communicated by the roots of plants 

 to pure water, probably came either from the continual 

 pushing oif of the tips of the rootlets by the interior 

 growing point — a process always naturally accompanying 

 the growth of roots — or from the disorganization of the 

 absorbent root-hairs. 



Under certain circumstances, small quantities of mineral 

 salts may indeed diffuse out of the root-cells into the water 

 of the soil. This is, however, no physiological action, 

 but a purely physical process. 



Vitality of Roots. — It appears that in case of most 

 plants the roots cannot long continue their vitality if their 

 connection with the leaves be interrupted, unless, indeed, 

 they be kept at a winter temperature. Hence weeds may 

 be effectually destroyed by cutting down their tops; al- 

 though, in many cases, the pi'ocess must be several times 

 repeated before the result is attained. 



The roots of our root-crops, properly so-called, viz., 

 beets, turnips, carrots, and parsnips, when harvested in au- 

 tumn, contain the elements of a second year's growth of 

 stem, etc., in the form of a bud at the crown of the root. 

 If the crown be cut away from the root, the latter cannot 

 vegetate, while the growth of the crown itself is not 

 thereby prevented. 



As regards internal st)~ucture, the root closely resembles 

 the stem, and what is stated of the latter on subsequent 

 pages, applies in all essential points to the former. 



THE STEM. 



Shortly after the protrusion of the rootlet from a ger- 

 minating seed, the Stem makes its appearance. It has, in 

 general, an upward dkection, which in many plants is per- 



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