On the Structure of Roots. 481 



this is exh.-! isted another root assumes the same form, to provide 

 for the succeeding^ year ; so that there are here always two 

 tuberous roots in the group of fibrous roots at the base of the 

 stem. In the day-lily {Hemerocallis) a number of roots become 

 thickened at their ends into large knobs in autumn ; while in the 

 asparagus all the cylindrical fibrous roots become enlarged and 

 fleshy, resembling pieces of macaroni. It is not worth while to 

 multiply examples ; most of these roots have a short period of 

 existence allotted to them, generally corresponding to that of the 

 annually renewed flowering stems, the difference being, that those 

 roots which act as store-roots are formed in autumn for the next 

 year's flowers, while in bulbs and corms, the roots, developed with 

 the leaves, are usually devoted to the supply of the nourishment 

 required by the new bulbs formed after flowering. In either 

 case their duration is connected essentially with one generation 

 of stem- buds. In the less familiar cases, where Monocotyledons 

 are arborescent, as in Palms, and in the more accessible case of 

 the Yucca, or Adam's Needle of our gardens, the roots are of 

 longer duration, and not only acquire considerable size, but 

 become woody. They still retain, however, their cylindrical, 

 blunt-ended character, and even here die away after a certain 

 time, being replaced by others developed above and outside them 

 at the base of the stem. 



So far we have spoken of roots growing in the ordinary condi- 

 tion. It remains to notice some unusual cases, such as that of 

 aerial roots, produced by plants growing suspended or supported 

 in the air, without either soil or water, and nourished by 

 atmospheric vapour alone ; of which the orchids are familiar 

 instances. These have, as might be expected, a peculiar structure 

 of the absorbing surfaces ; but in other respects resemble ordinary 

 roots. Then, again, there are the roots of real parasites, which 

 insinuate themselves into the stems or roots of other plants, 

 and abstract their nourishment wholly or in part from them. 

 This parasitical habit is accompanied by other remarkable 

 peculiarities, throughout the oiganization, of no practical im- 

 portance for our present purpose ; therefore it will be most 

 convenient to proceed now to one of the most important 

 branches of our subject, namely, the anatomical structure of 

 roots, which is the only safe basis for a rational inquiry into the 

 physiological laws presiding over the growth and development 

 of these organs. 



The root, as developed in the great majority of plants, presents 

 a highly-organized structure, made up of various kinds of true 

 cellular or parenchymatous tissue, together with those kinds of 

 elementary tissue which, under the names of wood-cells, vessels, 

 and ducts, form the hard parts of plants. As a rule, we may 

 divide the internal structures of a root into two regions — the 



