Buds and Stems. 55 



72. Make thin cross sections of stems of elm, cotton- 

 wood, or other woody plants which were placed in alcohol 

 in late summer, and treat with iodine. Can reserve food 

 in the form of starch or proteids be made out? (The 

 starch would be colored blue by the iodine, and the pro- 

 teids from yellow to brown.) Compare with these sec- 

 tions others of stems of the same plants taken at the time 

 of unfolding of the buds in the spring. What changes 

 have taken place in the reserve materials .■■ 



DISCUSSION. 



37. Upward Growth of the Shoot. — We have noticed that 

 in the germination of seeds the shoot grows straight upward 

 into the sunhght and air just as uniformly and persistently 

 as the roots grow downward into the soil, and it might be 

 inferred from this alone that the upward growth of the 

 shoot is just as necessary to the well-being of the plant as 

 the downward growth of the root. It has been noticed 

 that as the shoot reaches the surface, either the cotyledons 

 spread out in the form of thin green leaves, or the first 

 leaves of the plumule bud quickly unfold and place their 

 broad surfaces more or less at right angles to the Hght 

 from the sky, as in the case of the Lima bean, whose 

 cotyledons are so gorged with food materials that they 

 are prevented from developing into useful fohage leaf 

 forms. As the stem continues in its elongation, it puts 

 forth new leaves until it is nearly concealed by them and 

 is apparently subordinate to them. It may, in fact, be 

 stated that the chief function of stems is to bear leaves, 

 in such positions and at such distances apart as to give 

 them free access to the sunlight and air, and to keep them 

 in communication with the water and other raw food ma- 

 terials which are absorbed by the roots. 



