ii8 Introduction to Botany. 



to year, and new additions to the water-conducting ele- 

 ments and to the strengthening elements are not so neces- 

 sary as in the case of branching dicotyledonous stems. 

 Accordingly, we find that most monocotyledonous stems 

 increase but little in thickness, and give no evidence of 

 rings of annual growth, since the bundles are scattered, 

 and there is no true cambium ring. The grass stem, which 

 represents in its general structure a vast number of endoge- 

 nous stems, is a marvelous example of architectural achieve- 

 ment, for its height is often five hundred times its 

 diameter. If Washington Monument were built in like 

 proportions its base would cover an area of less than one 

 square foot. Stems of this sort, which increase but little 

 in diameter, show us how perfectly plants are constructed 

 from the purely mechanical standpoint. 



91. Sensibility of the Protoplasts. — One of the most re- 

 markable things about the live part of the cell, namely, the 

 protoplast, is its sensibility to its surroundings, and its 

 capacity to respond in certain definite ways to varying 

 external conditions. The polarity of plants, as we see it 

 exhibited in root and shoot, is an expression of this sensi- 

 bihty, since the protoplasts of the cells of the root use 

 gravity to guide their part of the plant body downward, 

 while those of the shoot find the upward direction by the 

 same force. The stem of the trumpet creeper finds its 

 way to a support through the perception of light, while the 

 leaves are directed away from the support and toward the 

 light by the same means. 



If, by any accident, a plant becomes overturned or bent 

 out of its normal position, the protoplasts perceive the 

 altered relation to light and gravity, and cause the growing 

 members to shift their positions into proper relations to 

 these forces. Figure 50 is a photograph of a young castor 



