SECT. I 



MORPHOLOGY 



131 



(Fig. 139 ex). Frequently some of the cells of such an exodermis remain un- 

 cutinised and serve as transfusion cells. They may be characterised by their 

 smaller size, and be regularly distributed between the cutinised cells. In aerial 

 roots the epidermis usually forms a many-layered root-slieath or velamen (cf. p. 120), 

 within which comes the exodermis. 



The primary cortex of the root is composed of colourless tissue, 

 which is usually parenchymatous. In the outer layers the cells are 

 in close contact with one another, but intercellular spaces are present 

 more internally. These intercellular spaces often widen into air- 

 cavities or passages. In many roots a hypoderma giving mechanical 

 support to the epidermis or exodermis is present. The outermost 

 layer of cells of the central cylinder 

 (Figs. 134, 1-41 p) forms the peri cycle, 

 which is also called the pericambium ; 

 this is usually a single layer, and in 

 rare cases is wanting. The xylem and 

 phloem portions form separate strands 

 (p. 124), radially disposed and alter- 

 nating with each other (Figs. 134, 140). 

 It has already been shown that the 

 narrowest elements of the vascular 

 strand are outermost. Roots are de- 

 scribed as diarch, t riarc h, p olyarc h. 

 according to the number of the vas- 

 cular strands. For example, the roots 

 of Acorns Calamus (Fig. 134) are 

 octarch, those oi Allium Cepa (Fig. 1 40) 

 hexarch. The vascular strands may 

 either. meet in the centre (Fig. 140), 

 or they may surround a central pith 

 (Fig. 134). 



Lateral roots are developed in 

 Pteridophytes from the innermost 



layer of the cortex, in Phanerogams from the pericycle of the parent 

 root. They have thus in their further development to break through 

 the whole thickness of the primary cortex of the parent root. They 

 are situated either directly opposite to the vascular strands of the 

 parent root, or between the xylem and phloem strands. The number 

 of rows of lateral roots is, therefore, as Van Tieghem (^'^) pointed 

 out, either equal to or double the number of vascular strands. As 

 the strands of the vascular bundles of roots take a straight course, 

 the lateral roots must similarly form straight rows. The distances 

 between the rows themselves are either ecpial, or, when the lateral 

 roots are situated to the right and left of each vascular strand, the 

 rows are arranged in pairs with wider intervals between each pair. 



The Leaves are composed of fundamental tissue (which is here 



Fig. 141. — Part of a transverse section of a 

 root of Iris Florentina. e, Enrlodermis, 

 showing cell walls thickened (jii one 

 side; /, transfusion cell; p, pericycle.; 

 V, phloem ; s, vessel of xylem ; c, cortex. 

 (X 240.) 



