46 



BOTANY 



PART I 



tri-radiate, l)ut in their character of leaves still subtend axillary 

 shoots provided with foliage leaves. By a similar metamorphosis, the 



two stipules of the leaves of the common 

 Locust (liobinia Pseudacacia) become modified 

 into thorns, while the leaf lamina persists as 

 a foliage leaf (Fig. 50). 



The Root. 



acacia, n, 

 into thorns ; 

 nat. size.) 



The third primary member of the cormo- 

 phytic plant in its typical development as an 

 underground root presents less marked difl'er- 

 ences in external form than were shown by 

 the other members. This may be put in 

 relation with the uniform conditions to which 

 _.„„,„. , roots are exposed in the soil. Certain difter- 



Fio. oO.— Part of stem anil com- ^ <■ i i i 



pound leaf of Rohinia Pseud- cuces are, however, fouud between the roots 

 'I, stipules modified of plants adapted to live in diflerent situa- 

 ns, <7, pu vmus. (A ^|Qj-|g_ 'pj^g j.^^^ ^^^ ^g jj-g n;,Qgt important 



function the absorption of water and nutrient 

 substances dissolved in it from the 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 provided with special 

 arrangements for aeration Avhich lead to localised swellings ; they do 

 not, however, undergo any fundamental change of form. Those roots 

 which grow in the air, aerial ROOTS, tend to 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 aftords 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 caps on 

 the water roots of Duckweed {Lemna) are not, in reality, I'oot-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. 2G) affords another example 

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



