48 PLANT BIOLOGY 



61. Experiments to show the upward path of sap through 

 stems. Laboratory Study No. 33. 



A. Stand some live twigs (e.g. maple or horse-chestnut) in 



red ink for a day or two ; cut off pieces above the 

 level of the ink, and split some of these pieces in 

 half lengthwise. 



1. Describe the preparation of the experiment. 



2. Through what part of the stem does the red ink rise ? 



3. What do you conclude, therefore, as to the part of a 



woody stem through which sap rises? 



B. (Optional.) Stand in red ink some pieces of fresh corn stalk 



(or if this cannot be obtained, some Tradescantia or 

 any lily stem). Cut some cross and longitudinal sec- 

 tions above the level of the ink. 



1. Write an account of the experiment, stating your observations. 



2. In the stem you are studying, is sap carried upward by the 



rind (epidermis in the lily), or by the pith, or by the 

 woody fibers ? How do you know ? 



62. Stems as organs for support and leaf exposure. 



When we studied the manufacture of carbohydrates by plants, 

 we proved that green leaves must be exposed to sunlight in 

 order to carry on this important function. When the leaves 

 receive the proper amount of exposure, the food can be 

 manufactured rapidly. Hence, we should expect to find that 

 leaves are arranged in such a way as to secure the best 

 amount of sunlight. Where plants are more or less crowded, 

 as in forests or thickets, the main stems, such as the trunks of 

 trees, usually grow tall, thus lifting the leaves to the light. 

 The amount of light exposure of trees and of most other plants 

 is largely increased by branches and their subdividing twigs, 

 ^to which the leaves are attached. 



In order that the trunk and its branches may be able to 

 support the leaves and withstand the force of storms, thick- 



