Ch. IV, 0] TRANSFER THROUGH PLAXTS 151 



after reaching their heights, tend to spread out laterally and 

 become flat topped ; and the dead branches which occasion- 

 ally occur above theii' green summits represent certain ones 

 which were able to exceed the ordinary limit in an especialh' 

 wet summer, but died in a dryer one. Trees which attain 

 to the greatest heights are apparently such as have par- 

 ticularly favorable structural relations to conduction or 

 transpiration. 



It is well knoT\Ti that in spring the sap exudes readily from 

 injuries in many kinds of plants. Thus Grape Mnes if 

 pruned too late in the season will ''bleed" very copiously, 

 and large drops of sap often fall upon sidewalks from broken 

 twigs or bark of shade trees : and the flow of sap in the Maple 

 in spring is a well kno\Mi phenomenon. These are e^•i- 

 dently cases in which the sap is forced out by osmotic pres- 

 sure in the roots, complicated, however, by osmotic pres- 

 sures in the stem, and liy expansion and contraction, under 

 varying temperatures, of the air which occurs in the stem. 

 The internal pressure developed liy such expansion explains 

 why the flow is greatest on a warm day after a cold night. 



The transfer, or more technically, traxslocatiox, of food 

 through the plant occurs only in solution in water. In 

 general the food has three paths. First, it may pass directly 

 from cell to cell through protoplasm and walls by the power 

 of diffusion, later to be studied. This is the sole method in 

 the lower and simpler plants, and is that by which the food is 

 removed from the chlorenchj-ma cells of the leaf to the near- 

 est veinlets (page 32). It is also the method by which the 

 food spreads from the ends of the veins to the growing cells 

 in all parts of the plant. Second, in woody plants in spring 

 the food stored in the roots or lower part of the stem is trans- 

 ferred into the ducts (along the medullary rays from the 

 bark), where it is rapidly lifted to the growing leaves -nith the 

 rising water current. This explains the presence of sugar 

 in the sap of ]\Iaple and other trees in spring, though later 

 in the season the sap is nothing other than soil water. Third, 



