i64 STEMS 



(See Figure 30.) Stems do not have this arrangement. 

 The growing tip of a root is covered by a protecting cap. 

 The growing tip of a stem has no such protecting cap. 

 Stems are jointed ; roots are not. Stems have outgrowths 

 called leaves ; roots have no such outgrowths. The 

 branches of roots arise at the edge of the stele and burrow 

 through the cortex to the epidermis. The branches of 

 stems arise from the outer part of the cortex, at the axils 

 of the leaves. Roots are positively geotropic, positively 

 hydrotropic, and negatively phototropic. Stems are usually 

 negatively geotropic, negatively hydrotropic, and positively 

 phototropic. 



B. Epidermis, Cortex, and Stele. — It is the arrange- 

 ment of the tissues of the stele which is the most compli- 

 cated feature of stem structure, and to that feature we 

 must give especial attention. As compared with the stele, 

 the epidermis and the cortex are simple and show little 

 variation. Most of the cells of the cortex, as you remem- 

 ber, have thin walls and nearly equal diameters. Tissue 

 composed of cells like this, whether it be in the cortex 

 or elsewhere, is called parenchyma. (The word means 

 parent-tissue.) All undifferentiated young cells are "pa- 

 renchyma; some remain parenchyma, others change to 

 tissues of other kinds. Parenchyma is the name of a tis- 

 sue as judged by its appearance rather than by its posi- 

 tion ; thus most of the cells of the cortex are cortical 

 parenchyma; the loose cells inside the leaf are spongy 

 parenchyma, etc. 



You will recall that in the cells of the older epidermis of 

 the root a substance called cutin is manufactured and that 

 this substance renders the cells which contain it imper- 



