THE TISSUES OF THE STEM 



55 



another in special cells (Fig. 33). The whole bundle is often enveloped in 

 mucilage which, swelling on any perforation of the cell from without, ejects 

 the crystals. Being sharply pointed they are found to serve as a protection 

 to the plants in which they occur against gnawing by 

 snails, and other animals. They are chiefly seen in the 

 Monocotyledons. 



Secretory cells or cell-fusions containing essential oils 

 are characteristic of certain families. For instance, 

 those borne on the glandular hairs of the Labiatae, 

 which are the source of their aromatic scents : or the 

 sunken cysts seen in the Rutaceae : those in the rind 

 of the Orange are good examples of these bodies. 



Woody Stems of Dicotyledons. 



The apical bud is potentially capable of un- 

 limited growth and production of new leaves ; the 

 bud in the axil of each leaf may also grow into a 

 branch similarly endowed. Correlated with these 

 developments the mechanical and conducting 

 tissues of the shoot are augmented by radial 

 growth. This is particularly evident in plants 

 which continue growing for a number of years, and 

 thus attain large size. In them the increase in 

 girth takes place by secondary growth, through the 

 activity of the tissue called the Cambium. Cam- 

 bium is also present in herbaceous Dicotyledons, 



1* 



J 



Fig. 33. 

 Cell of the 



cortex 

 of Dratana rubra, rilled 



but it is soecially active in enlarging the trunks and with mucilaginous matter 



1 and containing a bundle 



branches of shrubs and trees, rrom the bulky of raphides, r. (xi6o.) 



, r ... . . . r ■ 1 (After Strasburger.) 



column formed by its activity the superficial 



tissues may be peeled off, separating with special ease in the spring. 



The line of easy rupture is the cambium itself, and the reason why 



it splits so easily is that in the spring it is actively growing, and its 



cells are then thin-walled and weak. It will be necessary first to 



examine this tissue in detail, since it produces such important 



changes. 



In herbaceous stems of Dicotyledons, such as the Sunflower, Fig- 

 wort, or Cucumber (Figs. 23, 25), the cambium is seen between the 

 wood and bast of the vascular strands. In wood' stems it occupies 

 a similar position, as in the Elm (Fig. 34). The difference is only 

 one of the proportion of the tissues, and of the activity of the 

 cambium. Where the strands are separate, as in herbaceous stems, 

 the cambial activity may be seen to bridge over the spaces between 



