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 Dzcotyledons, 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): 
(1) an outer protecting lay- 
er which may be stripped 
P off as a thin skin, the epi- 
¢ dermis; (2) within this a 
zone of spongy tissue, usu- 
ally green, the corter; (3) 
then a relatively broad zone 
of firm wood, the vascular 
= cylinder; and (4) in the cen- 
eee en tie i, The eabeal 
cortex; w, vascular cylinder; p, pith. 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, 
