MORPHOLOGY 41 



tion. This regularity of form is due to the uniformity of the conditions 

 to which roots are exposed in the ground, for aerial roots, which are 

 for the most part restricted to the moist climate of the tropics, exhibit 

 a much greater tendency to modification. The covered vegetative cone 

 and the inability to develop leaves are characteristic of roots, and 

 furnish an easy means of distinguishing them from underground shoots. 

 A ROOT-CAP or CALYPTRA affords the vegetative cone of a root the 

 protection that is provided to the apex of a stem by the rudimentary 

 leaves. Although, generally, the existence of a root-cap is only dis- 

 closed by a median, longitudinal section through the root-tip, in some 

 roots it is plainly distinguishable as a cap-like covering. The very 

 noticeable caps on the water roots of Duckweed (Lemna) are not, in 

 reality, root-caps, as they are not derived from the root, but from a 

 sheath which envelops the rudimentary root at the time of its origin. 

 They are accordingly termed root-pockets (Fig. 415, wt). As a 

 general rule, however, roots without root-caps are of rare occurrence, 

 and in the case of the Duckweed the root-pockets perform all the 

 functions of a root-cap. The short-lived roots of the Dodder (p. 25) 

 afford another example of roots devoid of root-caps. Characteristic of 

 roots are also the root-hairs (Fig. 47, r), which are found at a short 

 distance from their apices. 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 other respects, root- 

 hairs, like prickles, show no regularity in their individual position or 

 number. In some few instances roots develop no root-hairs ; this is 

 true of the roots of many Conifers, and of most aerial roots. 



Branching of the Root. — Just as a 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 Lycopocliaceae, the shoots of which are also 

 dichotomously branched (p. 19). The branching of roots usually 

 occurs in acropetal succession, but the lateral roots (Fig. 47, 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. 47). They are of 

 endogenous 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, as with a collar. 



Adventitious roots, just as adventitious shoots, may arise from 

 any part of a plant. They are especially numerous on the underside 

 of rhizomes (Fig. 21, 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 

 leaves, especially from Begonia leaves. The origin of adventitious roots, 



