24 LECTURE III. 



solid, by the formation of wood ; as they become surrounded with a cork layer, they 

 cease entirely to take up nutritive matters and water from the earth : they are now 

 simply organs of attachment, and the function of nutrition is restricted to the younger 

 fibres of the root system. In tap-roots thickened to a turnip shape, which, espe- 

 cially by cultivation, swell up to huge, soft masses of tissue, a tissue, anatomically 

 similar to the wood but not lignified, also becomes formed in the ordinary thin 

 root-fil)res by the activity of a cambial layer : this causes the upper part of the 

 tap-root, with occasionally the lower part of the stem, to swell up in a napiform 

 manner. Common examples are Beet-root, Radish, Carrot, Chicory root, &c. 

 In this case the object is not only to attain greater solidity; these kinds of 

 roots are, rather, reservoirs of reserve materials, in which sugar, starch, or 

 inulin become stored together with proteid substances, which are employed in 

 the next period of vegetation for the production of new shoots. Here, also, the 

 function of absorbing nutritive matters remains to the lateral roots, which are 

 attached as fine threads to the large tap-root. Lateral roots may also swell up 

 to tuberous reservoirs of reserve material, as happens in the Dahlia, Hop, and 

 conspicuously in Ipomcea purga and Thladianiha dubia. In many other root-tubers, 

 the facts are otherwise ; the stout, round or lobed tubers of the Ophrydece, which 

 furnish salep, and the napiform tubers of the Monks' hood {Aconitum napellus) and 

 others, as well as those of Ranunculus Ficaria, are formed at the base of a subter- 

 ranean bud of the stem, and appear forthwith as thick, fleshy, short swellings, the 

 root nature of which is indeed not doubtful, which, however, together with the 

 other filiform roots of these plants, have essentially the purpose of serving as 

 reservoirs of reserve material for the shoot-buds connected with them. In the 

 species named, in fact, the whole plant disappears after the ripening of the fruit, 

 with the exception of these tubers, out of each of which a new plant arises in 

 the next period of vegetation. 



Of aerial roots, many may be regarded as quite ordinary typical roots, the full 

 development of which, however, becomes prevented by accidental outward circum- 

 stances. Thus, the aerial roots of the Ivy, which arise densely crowded and in 

 rows on the shaded side of the shoot axis, are capable, when supplied with earth, of 

 growing out into long, typical, branched roots, because darkness and moisture favour 

 their development ; usually, on the other hand, where they become too much dried 

 up in the air and hindered in growth by the light, they remain simple, short threads. 

 However, being sensitive to touch and at the same time negatively heliotropic, 

 they cling fast to the trunks of trees, rocks, and walls, on which the shoots climb 

 up, and so serve as clasping organs. Very instructive, with reference to the 

 capacity of the roots to adapt themselves to outward conditions of life, are also the 

 long aerial roots of some tropical Aroids (e.g. Monstera). These roots, arising in 

 the bud of the climbing stem, are vigorous, often 6-8 mm. and more thick, and 

 will develope into ordinary, branched, absorbing roots, if a shoot is cut off and stuck 

 into the earth. If, on the other hand, they are compelled to grow down through 

 the air, which sometimes occurs for several meters, they remain quite simple and 

 unbranched, until the apex finally penetrates into the earth, and there produces 

 a much-branched root system. If these aerial roots ha\e the opportunity of clinging 

 to a wall, a thick stem, &c., the apex soon becomes closely applied to it, and root-hairs 



