176 



OUTLINES OF PLANT LIFE. 



EXERCISE XLII. 



To show the effect of gravity on the growing regions of the stems of 

 grasses. 



Cover the bottom of a deep dish about 25 cm. long with a layer of wet 

 sand, and bank this against one end to the top. Into this bank stick 

 horizontally several grass stems having at least one node ; cover with a. 

 glass plate. After 24-48 hours observe curvature. Cut a longitudinal 

 section of the node and observe what part the leaf-sheath takes in this 

 curvature. 



248. . oot-cage. — Experiments upon the response of root- 

 lets to the stimulus of gravity when their position is altered 



1 \^\ may be carried on by means 



Xj^^/ of a root-cage. It consists 



~ ' essentially of two parallel 



panes of glass fastened to- 

 gether, between which, in 

 finely sifted soil, the rootlets 

 are grown. By inclining this 

 root-cage at various angles it 

 may be shown that not only 

 the primary root, but its 

 branches, strive to regain 

 their normal angle with the 

 direction of gravity. This is 

 illustrated in figure 130, in 

 which the dark portion of the 

 rootlets represents the grow- 

 ing parts while the cage was 



inverted. They then took about the same angle with the 



horizon as when in normal position. 



249. Twining plants. — The movements of twining plants 

 are due to a peculiar reaction to gravity. As the upper inter- 

 nodes of a seedling elongate they soon become too weak to 

 support themselves and bend over, becoming nearly horizon- 

 tal". When this occurs the growth of the right or left flank of 



Fig, 130. — Part of the root system of a broad 

 bean, grown in a root-cage, first in the 

 normal, then in the inverted, and again 

 in the normal position. The arrows show 

 the direction in wiiich gravity acted in 

 the different positions. The blaclc por- 

 tion of the roots were tlie parts growing 

 during inversion. Two thirds natural 

 size.— After Sachs. 



