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 ? 



EXPERIMENT 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, 

 as erect, prostrate, climb- 

 ing, inclined, declined, 

 underground, etc. 



91. Annuals complete 

 their life cycle in a 

 single season and then 

 die down as soon as they 

 have perfected their 

 seed. Many of our most 

 troublesome weeds be- 

 long to this class and 

 might be exterminated by the simple expedient of mowing 

 them down before their time of flowering. 



FIG. 92. Stems of red oak and hickory that 

 have grafted themselves. 



