SUPPLEMEN7 



And now we must consider the problem of how the current of water 

 with its freight is raised from the root-hairs to these cells in the leaves, 

 and the more one looks into investigations on this subject and reads 

 the various conclusions and theories of leading botanists, the less is 

 the inclination to offer any solution of the problem to children. 

 The old theory of capillarity seems to be quite abandoned. The laws 

 of the diffusion of fluids and gases separated by a membrane, such as 

 a cell wall, perhaps explain the great avidity of cells for water, espec- 

 ially of cells containing stored food and active protoplasm. At any 

 rate the water of the soil is drawn in by the root-hairs and forced on 

 by contiguous cells, and the force is named root pressure. Root 

 pressure is supposed to account for the rise of sap in maple trees in 

 the spring, the " weeping" of cut grape vines, the drops of moisture 

 often apparent on corn or wheat seedlings, and the gush of sap from 

 the cut leaves of the agave and other plants of arid regions. This 

 force is most active when stored food is being rapidly used in new 

 growth. Apparatus has been devised for measuring this force, and the 

 results can be found in any text book that treats to any extent of 

 plant physiology. 



There seems to be no doubt that the evaporation from the leaves is 

 a great lifting power. That is, the water given off by evaporating 

 cells near the su/face is replaced by diffusion from adjacent cells, 

 these in turn draw on cells below, and so on. In this way an ascend- 

 ing transpiration current arises. It seems to be quite proven that the 

 ascending current is mainly through the walls of the wood cells and 

 vessels, rather than through their cavities; and theories have been 

 advanced to meet this and other phenomena. After all, the main 

 point is that nutritive salts and nitrogen provided in the soil do reach 

 the laboratory cells in the leaves where they can be combined with 

 the organic material manufactured there. 



This manufacturing of organic matter from carbon dioxide and 

 water, or photo-synthesis, has perhaps been already sufficiently empha- 

 sized. As stated in the Reader, carbonic acid gas is absorbed directly 

 from the air; in the epidermal cells it has become carbonic acid and 

 the acid is absorbed and decomposed mainly by the protoplasm of 

 the palisade cells. The oxygen is of course given off through cell 

 walls into intercellular passages and thence through the stomata to 

 the outside world. The first product of photo-synthesis is not well 

 understood, and since in most plants it becomes visible first in the 

 form of starch in the chloroplasts, many teachers think best in ele- 

 mentary work to simply call the product starch and the process 

 starch-making. 



18 



