50 



A TEXT-BOOK OF BOTANY 



trunks, the tip of the tendril when it comes into contact 

 with a support is stimulated into developing the sucker- 

 like disk which acts as a holdfast (Fig. 50). 

 * 24. Internal structure. As the stems of seed-plants 

 show two distinct types of structure, it will be necessary to 

 point out the great groups of seed-plants, so that the types 

 of structure may be referred to them. The Gymnosperms 

 include the pines and their allies, the common evergreens; 

 the Monocotyledons include such plants as grasses, lilies, 

 and palms; the Dicotyledons, much the largest group, 

 include the common deciduous trees, such as oak, maple, 

 hickory, poplar, beech, etc., as well as the great majority 

 of common herbs. In stem structure the Gymnosperms. 

 and the Dicotyledons show the same general plan, while 

 the other type of structure is exhibited by the Monocotyle- 

 dons. 



(1) Gymnosperms and Dicotyledons. If an active twig 

 of an ordinary woody plant be cut across, it will be seen 



that it is made up of four 

 general regions (Fig. 51): 

 ~. W an outer Protecting lay- 

 er which may be stripped 

 'P off as a thin skin, the epi- 

 c dermis; (2) within this a 

 zone of spongy tissue, usu- 

 ally green, the cortex; (3) 

 then a relatively broad zone 

 of firm wood, the vascular 

 cylinder; and (4) in the cen- 

 ter the pith. The special 

 feature of this arrangement 

 is that the wood occurs as 



a hollow cylinder, enclosing the pith and surrounded by 

 the cortex. In the older parts of stems the pith often dis- 

 appears, leaving a hollow stem. The cortex is the active. 



W 



FIG. 51. Cross-section of a branch of box 

 elder one year old : e, epidermis ; c, 

 cortex ; w, vascular cylinder ; p, pith. 



