ROOTS AND THEIR USES. 



608 



soil, and a majority of short, Turnip-like forms in a stiff one. A similar 

 result followed in experimenting with Carrots. 



Hence, by the careful selection of seed from any form required, it 

 becomes fixed as a race, and the selected root is established by heredity. 



Root-Fibres and FIBRILS. — Secondary and subsequent rootlets issue 

 out of the primary root ; but they do not run vertically downwards, being 

 less influenced by gravity ; so that in a plant with a large number of 

 rootlets they are spread out and acquire a considerable volume. If the 

 primary root be cut away, then one or more of the secondary roots grow 

 downwards, just as, when a primary shoot of a tree is broken off, some 

 other bough rises up and becomes a leader instead of it. 



It should be borne in mind that it is only a very small portion — not 

 much more than one-twelfth of an inch — by which the root-fibre is 

 elongated. All behind that portion ceases to grow except in thickness. 

 Moreover, the absorbing power of a root-fibril is confined to the delicate 

 epidermal cells, whether they elongate into " root-hairs " or not, and is 

 equally transient, as the superficial cortical tissues are soon formed and 

 tin surface consequently becomes non-absorbent. 



The cultivator will thus clearly perceive the immense importance of 

 never breaking any of the finer root- fibrils when transplanting herbs, 

 as by so doing delay occurs in the plant having to form new ones, and 

 that delay may prove fatal. 



Adventitious Roots. — As the axial root may swell into various 

 forms, as in Radishes, so may the secondary as well as "adventi- 

 tious" roots, which issue out of stems, as in our terrestrial Orchids and 

 the lesser Celandine {Ranunculus Ficaria), Pfeonies, Dahlias, &c. Such 

 are reservoirs of nutriment ; but they become storage reservoirs of water 

 in very dry places, in order to sustain the plant during the intense heat 

 of the summer, as in deserts, &c. Some species of Heron 's-bill, for 

 example (Er odium), do this near Cairo : and our own Dropwortis probably 

 another instance, growing as it does in the very dry chalk hills. 



Contractile Roots. — As the underground stems of plants, such as 

 the root-stock of Primulas, the tubers of Arum, the corms of Crocuses, 

 and the bulbs of many monocotyledonous genera, require to be at certain 

 depths below the soil, according to the peculiarities of each kind of plant, 

 certain roots — and indeed the primary root itself of the Carrot &c. — have 

 " contractile " powers, whereby they pull the organ in question down- 

 wards. It is done by a shortening of the superficial cells ; the result is 

 seen in the horizontal wrinklings on the surface.* 



Endophytic Fungi. — A curious fact has been lately investigated with 

 Orchids. It is that their seeds on germination, as well as their roots and 

 rhizomes, are infested with the mycelium of fungi, species of Fusi- 

 sjjorium. 



Although it would seem that the fungus is a parasite and not living 

 symbiotically with the host — that is, for their mutual benefit— yet, as the 

 embryo of the seed of Orchids is always arrested in the pro-embryo stage, 



* For fuller details the reader is referred to a paper by Prof. Oliver, Joum. R. Hort. 

 Soc. vol. xxi. p. 486. 



