THE TISSUES OF THE STEM 43 



interrupted here and there by breathing pores, or stomata ; but none 

 of these happen to have been traversed in the section drawn. They 

 serve for ventilation of the tissues, and will be described later 

 (compare Figs. 47, 48, 51). 



Beneath the epidermis is a band of cortex, limited internally by the 

 endo dermis, which is the innermost layer of its cells. They are easily 

 recognised while young by the starch-grains which they contain (dark 

 blue with Iodine solution). The outermost layer of the cortex consists 

 of thin-walled cells containing green chloroplasts, while triangular 

 spaces occur where their walls meet. It is chiefly this layer that 

 gives the green colour to the stem. Beneath it lies a broad band of 

 cells with their cellulose walls thickened at their angles, which gives 

 them added strength. It is called collenchynta, and it passes over 

 gradually to an inner, thin-walled cortex, with cells of larger size, and 

 triangular intercellular spaces. These spaces are formed by splitting 

 of the cell-walls as they pass from the young to the mature stage, at 

 the angles where they meet. Since in life they are filled with gases 

 they form a connected ventilating system, which communicates with 

 the outer air through the stomata. The cells of the cortex are all 

 living cells, each with its protoplasmic lining. Gaseous interchange 

 is thus provided for them by the connected system of air-channels. 

 But this communication is cut off inwards, or at least restricted, by 

 the layer of the endodennis, in which the cells fit closely together, and 

 form a barrier between the outer tissues and those which lie within. 

 Thev are thin-walled, and in addition to the starch which thev contain 

 while young, they are characterised by a band of corky substance on 

 each radial wall, which being thrown into folds as the wall shrinks 

 on death of the cell, gives the appearance of a dark dot. The endo- 

 dermis is actually the innermost layer of the cortex, and it delimits 

 the tissues that lie within. The whole aggregate of these may be 

 styled the central cylinder, or more briefly the stele. 



The cortex is a variable tissue. The Potato gives an example of the bulk and 

 character that is usual for herbaceous stems, in which the stele is relatively 

 large and the cortex narrow. In young woody stems the stele is smaller 

 in proportion at first, and the cortex usually broader, as it is in Clematis 

 (Fig. 21). Aquatic plants have a contracted stele, and the cortex is more 

 bulky still, with large intercellular spaces ; as in Hippuris, or the Pond Weeds. 

 On the other hand, in some Monocotyledons, and especially in the Grasses, 

 it may be extremely narrow, so that the stele takes up nearly the whole trans- 

 verse area (Fig. 29). It may also vary in constitution. Sometimes it consists 

 wholly of soft green parenchyma, as in Hippuris : or this tissue may be 

 associated in various ways with strengthening collenchyma : or sometimes 



