180 COLLEGE BOTANY 



yet we have no entirely satisfactory explanation of this phe- 

 nomenon. In general, it may be said to be due to osmosis, dif- 

 fusion, capillarity, cohesion, transpiration and other factors. 



There is no true circulation of water or other fluids in the 

 plant comparable to the circulation of the blood in animals, but 

 in general it may be said that the plant fluids follow more or less 

 definite courses. The water passes upward through the xylem 

 of the fibro-vascular bundles, carrying more or less soluble sub- 

 stances, more especially the mineral constituents. There is also 

 a gradual downward movement of the soluble organic constitu- 

 ents through the protoplasmic contents of the sieve tubes and 

 from them, by diffusion, into the actively growing cells.. 



In the lower land plants, such as the liverworts and mosses, 

 the tissues are simple and the water travels for short distances 

 and its movements can be explained by osmosis. But in the 

 larger plants, in which the distances for transport are great, 

 the tissues are correspondingly complex and highly differen- 

 tiated and the transport much more difficult to explain. 



Girdling. Trees will live, after being girdled, for the re- 

 mainder of the season, because the flow of water and dissolved 

 food materials is unaffected by the girdled part, but will die 

 during the winter, because the girdling prevents the downward 

 flow of the protein materials which are normally stored in the 

 roots for use the following spring. The girdling of a branch of 

 a fruit tree or vine will prevent the downward flow of protein 

 through the phloem and cause it to be diverted to the fruit, which 

 becomes larger than normal. These fruits are said to be forced. 

 The same result is frequently brought about by winter injury of 

 trees at the surface of the ground ; some of the cortex and phloem 

 are killed, which results in a partial or complete girdling. 



