PRIMARY TISSUE REGIONS 131 



for the roots of various seed plants (cf. Ilayward, pp. 44-47), 

 but it is evident that this region of the root deserves further 

 intensive study with the aid of modern botanical microtechnique 

 (cf. von Guttenberg, 1940, 1941). The essential point here is 

 that the activity of the terminal raeristem of the root is funda- 

 mentally different from that of the shoot apex. In the latter, 

 exogenous leaf primordia arise from the flanks or base of the 

 apex. But in the root, two dissimilar patterns of differentia- 

 tion originate from the terminal meristem, one leading to the 

 outward addition of new cells to the root cap, the other con- 

 tributing new cells to the main but unsegmented body of the 

 root. These differences are at present impossible to explain but 

 it is evident that they determine the fundamental morphological 

 differences between root and shoot (cf. Arber, 1941). 



(b) Primary tissue regions. Young roots, prior to secondary 

 growth, resemble stems in the presence of epidermal, cortical and 

 stelar regions. The epidermis of roots is usually devoid of a 

 cuticle and stomata and its chief role appears to be that of 

 absorption, a process which is favored by the development, behind 

 the region of elongation, of a zone of root hairs. The cortex of 

 roots is often entirely composed of thin-walled storage paren- 

 chyma and is soon destroyed if secondary growth occurs. Appar- 

 ently the endodermis, which is commonly regarded as the inner- 

 most layer of the cortex, is a consistent feature of roots, in con- 

 trast to its variable distribution in the stem. But without doubt 

 one of the most fundamental characteristics of the young root 

 is shown by the arrangement and development of the primary 

 vascnlar tissues of the stele. In striking contrast to the collateral 

 position of xylem and phloem in the siphonostele of typical stems, 

 these tissues are arranged in a radial and alternate pattern in the 

 root. Thus, in the anatomical sense, there are no true primary 

 vascular bundles in the stele of the root. As seen in trans-sec- 

 tional view, the primary xylem and phloem of the root appear 

 as separate and alternating strands of tissue. Very frequently, 

 the xylem "plates" as they are often termed, meet in the center 

 to form a solid core. In many roots, however, particularly in 

 monocotyledons, the center of the stele is occupied by a core of 



