DIV. I 



MORPHOLOGY 131 



The bostryx and cincinnus are readily understood by deriving them from the 

 ground plan of the dichasium (Fig. 155 E). 



Various types of branching are frequently combined in one branch-system. 

 Thus cymosely-branched lateral shoots may be borne on the racemose main shoot. 

 The combinations are especially varied in the case of inflorescences. 



(b) The Root 



The ROOTS of plants, which are usually situated in the soil (subter- 

 ranean roots) and less commonly exposed to the atmosphere (aerial 

 roots), NEVER BEAR LEAVES. In this respect, as well as by the absence 

 of the green colour, their appearance differs from that of shoots ; even 

 of colourless subterranean shoots. Their chief functions are to attach 

 the plant to the soil and to absorb from this water and salts that 

 are conducted to the shoot-system. The functions of roots are thus 

 very different from those of most shoots, which mainly serve for the 

 assimilation of carbon dioxide. 



1. Growing Point. The root grows in length at the tip, exhibiting 

 APICAL GROWTH by means of its conical GROWING POINT. The latter 

 requires to have the thin-walled meristematic cells specially protected 

 since, as the root grows, it is forced forwards like a needle between the 

 angular particles of the soil. This protection is afforded by a special 

 organ composed of permanent tissue which is called the ROOT-CAP or 

 CALYPTRA ; it covers the tip of the root as a thimble does that of the 

 finger, the true growing point having an intercalary position within 

 the tissue of the root-tip. The outer cell walls of the root-cap become 

 mucilaginous, and this makes the forward passage of the root easier. 

 The root-cap is usually only recognisable in median longitudinal sections 

 through the root-tip (Figs. 156, 157), but in some cases (Pandanus) 

 the cap is to be clearly seen on the intact root. 



The very noticeable caps on the water roots of Duckweed (Lemna) and of some 

 Hydrocharitaceae are not really 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-pocket performs all the functions of a root-cap. The short-lived root of the 

 Dodder (p. 190) affords another example of a root devoid of a root-cap. 



The growing point of the root, as has been already mentioned, is 

 composed of meristematic cells from which the permanent cells of the 

 root-cap are derived on the side towards the tip and the permanent 

 tissue of the root on the basal side. 



In most Pteridophytes the root, like the shoot, has a three-sided 

 apical cell (t, Fig. 156) with the form of a three-sided pyramid. 



In addition to the segments cut off parallel to the three inner walls which 

 contribute to the root itself, segments are formed parallel to the outer wall (). 

 These undergo further divisions and form the root-cap. 



K 1 



