218 MORPHOLOGY. 



Epiblema and vascular bundles are formed in the root in the 

 manner already described; the latter are so placed that they pre- 

 sent a closed circle in the cross -section. In Monocotyledons they 

 are closed or definite bundles; in Dicotyledons indefinite. They 

 enclose a minute pith. Liber-bundles, milk-reservoirs, and milk- 

 vessels are sometimes formed in the bark. 



The distinction between main root in the immediate elongation of the 

 radicle, and branches of the root which issue from that subsequently, is 

 the only one morphologically essential ; on the other hand, it is neces- 

 sary, in a physiological point of view, as will be discussed hereafter, to 

 distinguish the simple, newest, and still advancing end, from all other 

 parts of the radical system. 



That every true root has a distinct, even though minute, pith, i. e. 

 a parenchyma enclosed by a circle of vascular bundles, is demonstrated 

 by every longitudinal and transverse section brought beneath the mi- 

 croscope. 



b. Adventitious Root (Radix adventitia). 



124. Adventitious roots are developed, in a peculiar manner, 

 from the axis, either under favouring external conditions (as, for 

 instance, a considerable degree of moisture, artificially as in cuttings, 

 naturally through the weak axis lying upon the ground as in the 

 so-called runners)', or with specific regularity, as in Grasses, 

 plants with aerial roots, &c., and from the true root, but here in 

 perfect regularity. In the bark, close upon the vascular bundles, 

 originates a little conical group of formative cells, which separates 

 quite down to the base of the cone from the surrounding cells, and, 

 taking on the process of growth peculiar to the root, breaks a way 

 for itself through the bark and becomes free. In this act it usually 

 compresses that portion of the cortical parenchyma lying in front 

 of it ; this dies, is torn away, and often remains adherent for a long 

 time upon the apex of the root as a little cap, as, for instance, in 

 Equisetum, Pandanus*, &c. This must not be confounded with 

 the calyptra of the root on the adventitious roots of plants rooting 

 in water, such as Lemna'f, Pistia, &c. 



In most of the tropical Orchidece, in many species of Pothos, the 

 adventitious roots, which may be developed either in the air or in 

 the ground, have a peculiar investment over their true epidermis 

 ( 29.). These appear to deserve a special name, and I call them 

 coated roots (radices velatce). 



When the adventitious roots are produced regularly upon those 

 internodes of a species of plant exposed to the air, they are named 

 (with a superfluous term) aerial roots (radices aerece). 



* According to DeCandolle, Organographie Vegetale, vol. ii. pi. 10. I have never 

 seen it in our hothouses. 



( Here we have a striking example of how senseless the terminology sometimes is. 

 The roots, hanging perpendicularly down in the water, of the floating Lemna are named 

 radices natantes. One might just as well talk of a swimming anchor, which, with 

 thirty fathoms of cable, does not yet reach the hottom. Such things never happen to 

 plain every-day people, only to a scholar who has wholly destroyed his healthy powers 

 of perception by book-wisdom and an in-door life. 



