82 PRACTICAL COURSE IN BOTANY 
thinner walled cells of the internal tissues. These, when relieved of the 
resistance of the thicker walled outer tissues, swell on their free side, but 
are held back on the other by the non-absorbent outer parts, as one side 
of the muslin ruffle was held by the gathering thread. 
EXPERIMENT 56. To FIND OUT WHETHER THE DIRECTION OF STEM 
GROWTH IS INFLUENCED BY LIGHT. — Place two rapidly growing young 
pea, bean, sunflower, or squash plants, each with several well-developed 
leaves, in a room or box with a light exposure on one side only. After two 
or three days, notice the position of the stems in regard to the light. Does 
either one show a more decided inclination toward it than the other? 
EXxpeRIMENT 57. Is THE LIGHT RELATION OF THE STEM INFLUENCED 
BY THE LEAVES ?—Cut the leaves from one of the plants used in Exp. 56, 
covering the cut surfaces with vaseline to prevent “bleeding”; reverse 
the positions of both with regard to the light, and watch for two or three 
days. In which is the response to light the more rapid? What does this 
indicate as one object of the stem in seeking light? What is the best 
position of a stem, ordinarily, for getting its leaves into the light? 
90. Classification. — Stems are classed according to 
(1) duration, as annuals, biennials, and perennials; (2) with 
reference to hardness or 
softness of structure, as 
herbaceous and woody ; 
(3) in regard to position 
and direction of growth, 
aserect, prostrate, climb- 
ing, inclined, declined, 
underground, etc. 
or. Annuals complete 
their life cycle in a 
single season and then 
die down as soon as they 
ce have perfected their 
ae ers | seed. Many of our most 
Fic. 92.—Stems of red oak and hickory that troublesome weeds be- 
have grafted themselves. ‘ 
long to this class and 
might be exterminated by the simple expedient of mowing 
them down before their time of flowering. 
