STRUCTURE OF ROOTS 35 



the cortex consists of closely-fitting cells with thickenings on 

 their radial and outer walls. This layer is the endodermis 

 (Gr. endon = within). The inner tissues form the cen- 

 tral cylinder or stele, and consist of a layer of delicate 

 cells next to the endodermis, called the pericycle (Gr. 

 peri = around, kyklos = circle) ; within this are the veins 

 or vascular bundles, each being composed of three kinds of 

 tissue an outer tissue, the bast or phloem (Fig. 15, b) (Gr. 

 phloios = bark) ; an inner tissue, the wood or xylem (w) 

 (Gr. xylon = wood) ; and between the two a very delicate, 

 actively growing tissue, the cambium (c). 



The water, which has been absorbed by the root-hairs, 

 is transmitted through the cortex to the wood, and through 

 this conducted upwards to the stem, to which the veins go 

 in the form of a complicated network. Excellent skeletons 

 consisting of the veins of a Radish can often be obtained 

 from a garden rubbish-heap. 



Tissues of a young root. The structure of a very young 

 root, however, is different (Fig. 15, 2). The bast is not 

 placed on the outside of the wood, i. e. collaterally, but the 

 two tissues are arranged in alternating groups, i. e. radially. 

 The groups of first-formed or primary wood (Fig. 15, 2, w, 

 and Fig. 16, r, 2, 3, p.w) develop from outside inwards (the 

 ends of the rays being the oldest parts), and by further 

 growth they form a solid mass of wood in the centre. In 

 such a root there is no pith. It is not until a cambium ring 

 is formed, winding to the outside of the wood and to the 

 inside of the bast, that the arrangement found in an old 

 root is developed. Fig. 16, 1, 2, 3, makes this point clear. 

 Secondary growth. From the cambium and by the divi- 

 sion of its cells to form new tissues, arises new wood to the 

 inside of it (Fig. 16, 2, 3, s.w), and new bast to the outside 

 (Fig. 16, 2, 3, s.b) ; and in this way a ring of vascular bundles 

 is formed, each bundle consisting of the three tissues 

 bast, cambium, and wood. At first the cambium is a wavy 



c 2 



