284 PLANT PHYSIOLOGY 



This kind of evidence, while not very scientific, is of interest in 

 connection with the other lines obtained. 



2. Analysis of the phloem contents. The phloem is extremely 

 rich in food materials which show it to be either a region of con- 

 duction or of storage. That it is not a storage region, is shown by 

 the fact that these foods are in solution rather than in a storage 

 form as is the case with the endodermis. An average analysis 

 during the period of active photosynthesis and storage shows 

 the phloem cells to contain about 30% of their dry weight in 

 carbohydrates and 60% in proteins and other nitrogen compounds. 



3. Distribution of the phloem. The phloem is found in greatest 

 abundance or with larger elements where there is greater need 

 for translocation. In vines with narrow, slender stems and many 

 leaves such as the grape and cucurbits, there is much need for an 

 easy movement of foods between the leaves and the parts below 

 as well as to the ripening fruits. The cucurbits not only have 

 extra large sieve tubes in the phloem, which makes them especially 

 good objects for classroom study, but they have phloem on both 

 sides of the xylem (bicolateral bundles). The breadfruit (Arto- 

 carpos) contains extremely large and numerous phloem elements 

 in the peduncles of its large fruits. Similarly in flower and fruit 

 clusters, where much food must be conducted in from outside, 

 the peduncles may have 50% more phloem than ordinary stems 

 of the same size. In these peduncles the movement is morpho- 

 logically upward instead of downward and supports the contention 

 of Curtis that upward movement also occurs in the phloem. 



4. The girdling experiments. In girdling or ringing, a narrow 

 strip of bark is removed from the stem or branch down as far as 

 the cambium. This strip will include the phloem, which will 

 therefore be interrupted for a distance equaling the width of the 

 removed strip. When the trunk of a tree is girdled, it is observed 

 that the tree ultimately dies but not before the roots become de- 

 pleted of the food stored in them. Upward conduction of water 

 continues without interruption, but the downward translocation 

 of materials is stopped at the girdle. Food accumulates above the 

 ring, and frequently the stem becomes much swollen at that point. 

 If willows or other similar plants which form roots readily, are 

 girdled and placed in water, roots will form above the girdle only 

 and not below. 



While girdling shuts off the transport of food in trees and shrubs, 



