SECT. I MORPHOLOGY 47 



ROOT-HAIRS (Fig. 173 r), which ;ire found at a short distance from 

 their tips. As the older root-hairs die at the same rate that the 

 new ones are developed, only a small portion of a root is provided 

 with root-hairs at the same time. In some few instances roots 

 develop no root-hairs ; this is true of the roots of many Conifers. 



Branching of the Root. — Just as the shoot may become bifurcated 

 by the division of its growing point (Fig. 18), so a root may become 

 similarly branched. For the most part, this mode of branching 

 takes place only in the roots of Lycopodiaceae, the shoots of which are 

 also dichotomously branched (p. 19). The branches of roots usually 

 occur in acropetal succession, but the lateral roots (Fig. 173 sw) make 

 their appearance at a much greater distance from the growing point 

 of the main root than lateral shoots from the apex of their parent 

 stem. By reason of the internal structure of their parent root, lateral 

 roots always develop in longitudinal rows (Fig. 1 73). They are of endo- 

 genous origin, and before reaching the surface must break through the 

 surrounding and overlying tissue of the parent root, by the ruptured 

 portions of which they are often invested at the base, as with a collar. 



Generally the lateral roots are reinforced by the development on 

 the parent root of specially strong adventitious roots. These arise 

 first on the older and later on the younger parts of the root, Avithout, 

 however, exhibiting a strictly acropetal succession. In Monocotyledons 

 the root-system of the embryo plant is at once replaced by adventitious 

 roots developed from the lower part of the stem. Adventitious roots 

 are generally endogenous ; they can, like adventitious shoots, be 

 formed on various parts of the plant. 



The place of origin of adventitious roots is not fixed beforehand but maj'- (as 

 is also the case with many adventitious shoots, p. 21) be more or less definite. 

 This is especially the case in marsh- and water-plants where the roots arise from 

 the lower nodes of the stem between, and alternating with, the leaves ; they 

 replace the primary root system which has been lost when the older part of the 

 l)lant died off ('-''). The further development of the rudiments of adventitious roots 

 is determined by the needs of the plant. They are especially numerous on the 

 under side of rhizomes (Fig. 23 w), and also, when the external conditions are at 

 all favourable, they seem to develop very readily from the stem nodes. A young 

 shoot, or a cutting planted in moist soil, quickly forms adventitious roots, and 

 roots may also arise in a similar manner from the bases of leaves, especially from 

 Bcfjonia leaves when planted in soil. Dormant root rudiments occur in the same 

 manner as dormant buds of shoots. Willow-twigs aft'ord a special case of the 

 presence of such dormant rudiments of adventitious roots, the further development 

 of which is easily induced by darkness and moisture. . When adventitious shoots 

 are borne upon roots they mostly arise at some distance from the apex. In the 

 Adder's Tongue Fern {OpMoglossimi), the vegetative reproduction of which is effected 

 by means of buds formed on the root, these arise close to the root-apex. Lastly, 

 just as terminal leaves may arise from the apex of a stem and terminal shoots 

 from the tips of the leaves of Ferns, in a few instances (species of As2)lenium and 

 PlufAj cerium a.\\\ong Ferns, Neottia nidus avis in the Orchidaceae) a shoot nuiy arise 

 as the direct continuation of the growing point of a root (-'). 



