ORGANS OF NCTRITIOX. 119 



Section 2. The Root or Descekdikg Axis. 



The root may be defined as that portion of the axis which passes 

 at its first development in an opposite direction to the stem, avoid- 

 ing the light, fixing the plant to the soil, or to the substance upon 

 which it grows, or floating in the Avater Avhen the plant swims 

 upon the surface of that medium, and deriving nourishment for 

 it. The part where the stem and root diverge has been called 

 the neck or collum, or formerly, the vital node, because it Avas 

 erroneously supposed to be the seat of the life of the plant. The 

 term coarcture has been also applied to the same point, from the 

 axis being there generally more or less contracted, at least in the 

 young plant; but, as it continues to develope all traces exter- 

 nally of this point arc usually destroyed, so that after a few years 

 it becomes very difficult, if not impossible, to discover its posi- 

 tion. That part of the root which joins the stem is called the base, 

 the opposite extremity the apex. Having so fully described the 

 structure and general characters of the stem, it will be unnecessary 

 for us to dwell further upon the root than to indicate especially 

 those points where it presents any distinctive appearances. 



Roots may be divided into two classes, namely. True or 

 Pinmary, and Adventitious or Secondary. Our remarks up to 

 the present time apply more particularly to the former, to which 

 we shall also at present continue to direct our attention. 



1. True or Primary Root. — This is formed at first by addi- 

 tions made within the exti-emity of the radicle of the embryo; and 

 the mode in which it takes place may be thus stated: — Growth 

 commences by the multiplication of cells by division, just within 

 the apex of the radicle {fig. 230, a); these cells then elongate 

 by their own inherent vitality, by which the tissue constituting 

 the apex b is pushed onwards, and gradu- p- q^q 



ally perishes, or is thrown off; the in- 

 nermost of these newly formed cells then ^-^ ~^ 

 remain unaltered, while others immediately ^'^TRl^Ilj^^ 

 within the point of the root continue to ^^^mWwK^ 

 multiply by division and grow in a similar f^^^^^^\^ 

 manner to the former, by which a layer of <f^mlW/^^'''\^ 

 tissue at the apex is again pushed for- -^drninW^ 

 ward and perishes in like manner; then "^IMi/wMi 

 new growth commences within this as in ^JH+mllJI^ 

 the former instance, to be followed by similar limfflf 

 changes; so that additions are always made ^^^ " 

 at first just within the extremity of the \^?^- i 

 root, and not directly at its extremity, as 

 is often stated, and* as new tissue is de- Fig. 230. Young root of 



, T ., . 1 n 1 i 1 rrn a Maple masnified. a. 



veloped it is gradually destroyed. Thus, xiie part wiiere growth 

 the apex of the root is ahvavs clothed by a >» taking place, b. The 



1 f -, ■ .1 ^i" ^ 1 • 1 • 1 original extremity. 



layer of denser tissue than that Avhich is be- After Gray. 



I4 



