160 PLANT PHYSIOLOGY 



and as building-material for protoplasm. Because latex — 

 the contents of the milk-tubes — is composed in part of such 

 nutritious substances as starch, sugars, fats and oils, and 

 proteids, and because the milk-tubes form a system wholly 

 uninterrupted throughout its extent by cross-walls of any 

 kind, the suspicion is irresistible that these tubes offer the 

 easiest course for the transfer of food from part to part. 

 They occur in plants relatively few in number and conse- 

 quently cannot be indispensable to food transfer. Their 

 functions too need further investigation. 



The opposite process to that which goes on in the chloro- 

 phyll-containing cells of the leaves, whereby the elaborated 

 carbohydrate temporarily deposited as starch is dissolved 

 for transport elsewhere, takes place in the organs where 

 carbohydrates are stored in solid form. In roots, rhizomes, 

 and tubers, in pith and medullary rays, and in seeds, 

 parenchyma cells remove the sugar from the solution in 

 which it comes to them by depositing it as starch grains in 

 the protoplasm or as cellulose lining their walls. How the 

 carbohydrates are acted upon by the protoplasm, or by its 

 special organs the leucoplastids, how sugars are converted 

 into cellulose or into starch and by this means are removed 

 from solution, are still unanswered questions. It is easy to 

 see that if sugar is removed from the cell-sap more will go 

 by osmosis to the cells removing it from solution. This 

 last is a necessary physical consequence of the physiological 

 (that is, in this case, of the combined chemical and physical) 

 action of the living protoplasm. But this storing action of 

 the protoplasm is as little understood as the first secretion 

 of sugar in nectaries (p. 126). 



Still less comprehensible at the present time are the ac- 

 cumulations in limited regions of the plant of substances 

 which ordinarily would move osmotically in one direction as 

 readily as in another. The storage of carbohydra^te in the 

 form of inulin, dissolved in the cell-sap of dahlia-tubers and 

 in the underground parts of other Compositse, is not to be 

 explained by the ordinary laws of physics. The living pro- 

 toplasm which, in one part of the plant, elaborates food 

 and permits its exosmosis, accumulates food and prevents 



