90 BOTANY. 



in length for a greater time, so that each internode may 

 grow after many others have formed above it. In such a 

 case the lower internodes are the first to cease growing, 

 and these are followed by those above in succession. The 

 increase in the height of a plant is the aggregate growth of 

 its internodes (Fig. 50). 



Practical Studies. — (o) Make longitudinal sections of the tip of the 

 root of Indian corn, or onion, and study in succession the cells of 

 difEerent ages, beginning at the growing point. Note the differences 

 between the young cells near the growing point and the older ones at 

 a distance from it. 



(J) Make a cross-section of a young (green) stem and observe that 

 all the cells are active in growth. 



(c) Make a cross-section of a one-year-old twig of a dicotyledon (as 

 apple, elm, or willow) and observe that the growing cells are confined 

 to a narrow ring, the cambium, between the wood and bark. 



(d) Study the growth of Indian-corn root by marking it at regular 

 intervals with India ink. 



(e) Measure the rate of growth (in length of stems) by means of an 

 auxanometer (Fig. 50), 



THE PHYSICS OF VEGETATION. 



159. Since all parts of plants are composed of matter, it 

 follows that they are subject to physical forces. In a living 

 cell there is no suspension of the action of any force or of 

 any physical law. Every atom of matter in the cell is as 

 much under the control of force as it was before it entered 

 into living matter. In each cell there are many active 

 forces, and what we see is the resultant of all, not of one 

 alone, and it is this complex result which sometimes has 

 puzzled us. It is impossible at present to make a complete 

 statement of all the physical activities in living plants; we 

 may, however, study the behavior of the living cells, cell- 

 masses, or the whole plant under the influence of physical 

 forces of varying intensities. 



