32 MORPHOLOGY OF SPECIES 



tissue, radiating from the central part of the spine, thickened on 

 the outer edge (Fig. 12, S). In the hollows between the ridges 

 we find two or more rows of thin-walled cells, full of chlorophyll 

 corpuscles. This is the assimilative tissue (Figs. 12 and 13, a). 

 The epidermis surrounding the stem possesses a very thick cuticle, 

 and the stomata are slightly sunk below the surface. The stomata 

 occur almost exclusively in the hollows of the stem and are pro- 

 tected by hairs (Figs. 12 and 13). Just inside the chlorophyll 

 layer is a single row of large thin-walled cells, with very little 

 contents and no chlorophyll corpuscles; next to these are the 

 endodermis and pericycle, followed by a ring of vascular bundles, 

 one to each ridge (Figs. 12 and 13). The structure of the normal 

 stem resembles this, but the chlorophyll tissue is not so well 

 developed, and secondary thickening takes place at an early stage, 

 so that a complete ring of vascular tissue is seen instead of sepa- 

 rate bundles. 



The presence of the assimilative tissue in the hollows of the 

 stem only has been explained as preventing a too-rapid evapora- 

 tion of water, such as takes place when large leaf-surfaces are 

 exposed. It has been suggested that this modification is therefore 

 a safeguard against drought, but Kerner is of opinion that it is 

 merely a contrivance to prevent the wetting of the stomata. That 

 this in part explains the modification is probable ; but it seems to 

 me that, in common with many other contrivances of a similar 

 nature — such as rolled-up leaves, needle-shaped leaves, etc. — it is 

 mainly a protection against excessive transpiration. But, so far as 

 I know, the Furze plant, although it is grown in a rather dry soil, 

 is never or very rarely exposed to excessive dryness, and therefore 

 this provision against drought may appear to be of very little use. 

 But drought is not the only cause of excessive transpiration. 

 Cold winds promote transpiration, and at the same time tends to 

 retard the absorption of moisture by the roots. The Furze plant 

 grows in exposed situations generally, and would thus be very 

 liable to be affected in this manner were it not for the reduction 

 of the leaves and the modification of the stem. In fact, even now 

 the F'urze is unable to stand very severe cold, and nearly all the 

 young developing buds are killed during the winter. But without 

 committing ourselves to one explanation or the other, we may, I 



