46 BOTANY i-.uir i 



the other members. This may be put in relation with the uniform 

 conditions to which roots are exposed in the soil. Certain differences 

 are, however, found between the roots of plants adapted to live in 

 different situations ( 25 ). The root has as its most important function t la- 

 absorption of water and nutrient substances dissolved in it from tin- 

 soil, and also serves to attach the plant firmly in the soil. Frequently it 

 serves for storage of assimilated substances. The general appearance 

 of an underground root differs as it is more specially adapted to perform 

 one or other of these functions. Those roots which grow in water or 

 mud become more elongated, are little branched, and are often pro- 

 vided with special arrangements for aeration which lead to localised 

 swellings ; they do not, however, undergo any fundamental change 

 of form. Those roots which grow in the air, AERIAL ROOTS, tend t<> 

 be more strikingly modified. 



The absence of leaves and the existence of a root-cap protecting 

 the growing point 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 leaves of the bud. Although, 

 generally, the existence of a root-cap is only disclosed by a median, 

 longitudinal section through the root-tip, in some roots it is plainly 

 distinguishable as a cap-like covering. The very noticeable cap- mi 

 the water roots of Duckweed (LemtM) 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. 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 root of the Dodder (p. 27) affords another example 

 of a root devoid of a root-cap. Characteristic of roots are also the 

 ROOT-HAIRS (Fig. 170, r), which are 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. 170 w) 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 70). They are of endo- 

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



