MUTUAL ACCOMMODATIONS BETWEEN PLANT ORGANS. 155 



different, but nevertheless they are fundamentally of the same 

 nature ; and though destined for different purposes, they can 

 both alike acquire the same functions. Thus roots normally fix 

 the plant into the ground and supply it with mineral matters in 

 solution, and with water. They may, however, become thick and 

 fleshy, and act as reservoirs of nutriment for future use, as in 

 biennial carrots and turnips, and the "tuberous roots" of the 

 dahlia. Occasionally, if they lie near the surface, or are 

 accidentally exposed to light, they can take on one of the 

 functions of stems, and produce leafy branches, as may be seen 

 in elms in a hedgerow ; the hedge being often entirely composed 

 of elm-saplings. The roots of plums and raspberries are par- 

 ticularly prone to throw up young plants, suggesting the hint 

 that to propagate any plant by its roots they should be raised 

 towards the surface of the ground or exposed. Eoots may 

 adhere to those of other plants, and become parasitic, as in 

 broomrapes ; and if exposed to light may turn green, and 

 become assimilative organs, as in some orchids. 



Stems. — These are usually the aerial structures of support, 

 and normally convey the fluids from the roots to the leaves. 

 Many, however, are partly subterranean, as mints, and then they 

 anatomically approximate the structure of a true root. If, e.g., 

 a living aerial branch be buried, as it continues to elongate, its 

 tissues at once commence to resemble those of a normally 

 subterranean stem, which is intermediate in structure between 

 an aerial stem and a root. Conversely if the underground shoot 

 of a potato reach the surface, instead of forming a tuber it grows 

 up into an erect stem and leafy branches. But if a potato is 

 entirely prevented from forming tubers, the tendency to do so is 

 so strong throughout the plant that it now forms them in the 

 axils of the leaves ; proving the homologous nature between an 

 ordinary aerial branch and a subterranean tuber. 



As stems (unlike most roots) normally produce buds, it is not 

 surprising to find subterranean stems and others upon the 

 surface of the ground perpetually propagating the plant by 

 means of them, as convolvulus and coltsfoot underground, and 

 strawberries, houseleek, &c, upon the surface. 



That stems can produce roots, even more readily than roots 

 can produce buds, is known to all gardeners who propagate 



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