168 THE GROWTH OF BOOTS AND STEMS. 



aniline chloride) is more strongly developed towards the outside of the 

 stem. 



In many Monocotyledon stems the outer bundles are embedded in a 

 layer of hard tissue which forms a complete tube ; this is well shown 

 in shrubby plants like Butcher's Broom and Asparagus. Cut sections 

 of the stems of these and of other Monocotyledons, also of 'the flower- 

 ing stems of Lily, Hyacinth, etc. In all cases the bundles are more 

 crowded towards the outside, and the hard tissue is developed as a 

 layer in the outer part of the ground-tissue. In the erect stems of 

 Monocotyledons, therefore, the "skeleton" or supporting tissue is 

 disposed in the form of a tube, in spite of the "scattered" arrange- 

 ments of the bundles. 



(b) Cut longitudinal sections of the Maize stem (or those of other 

 Monocotyledons), and notice the course of the bundles. This can 

 easily be done by cutting thick sections passing through the bases of 

 several leaves and placing them in caustic potash for some time. In 

 these plants, and in most other Monocotyledons, the leaves have a 

 broad insertion, and from them a number of bundles can be traced into 

 them. Their downward course in the stem is not parallel to the sur- 

 face, but curved. They first run obliquely downwards towards the 

 centre, and then bend outwards again towards the surface. After 

 running through one or two internodes, they join on to bundles passing 

 in from other leaves. Hence at all levels we have bundles situated at 

 varying depths in the ground- tissue, and so the transverse section of 

 the stem shows a " scattered " arrangement of the bundles. 



199. Tissues of the Herbaceous Stem. From the fore- 

 going work on stem-structure it is evident that the tissues of 

 a herbaceous stem (and the young parts of a woody stem) are 

 of three kinds : (1) epidermis, (2) ground-tissue, (3) vascular 

 tissue (wood and bast) . 



(1) The epidermis consists of a single layer of cells, with 

 thickened outer walls (cuticle), and bears stomates and often 

 hairs. It agrees closely with the epidermis covering the leaf, 

 and is continuous with it. 



(2) The ground-tissue consists mainly of thin-walled cells, 

 those in the centre of the stem often becoming dried up and 

 torn asunder, while those in the regions between and on the 

 outer side of the bundles have various important functions. 

 The cells in the outer region (cortex) usually contain chloro- 

 plasts, and are supplied with air which enters by the stomates 

 and traverses the spaces between the cortex-cells ; these cells 

 are therefore, like the cells of the ground-tissue (mesophyll) 

 of the leaf, able to make organic substance by photosynthesis. 

 The outermost cortical cells often become thick- walled, and 



