342 



PLANT PHYSIOLOGY 



The scheme is more complicated with the passage of the solu- 

 tion of organic substances either from the assimilating cells into 

 the sieve tubes or from the sieve tubes into the consuming cells, 

 and in general with the transfer from one Uving cell to another. 

 As has already been seen in Art. 5, living cells are almost com- 

 COo Hi H2O pletely impermeable to most dissolved sub- 



stances, this impermeabiUty being the cause of 

 the high osmotic pressure prevaiUng in them. 

 Sugars especially penetrate with difficulty into 

 the cells. Nevertheless, sugars move very easily 

 through the plant, and according to Miinch's 

 analysis they form the chief part of the sieve- 

 tube contents. Miinch finds the solution of this 

 difficulty in the plasmodesmen, the fine plasmatic 

 communicating threads that pass through pores 

 in the walls of the cells connecting all the pro- 

 toplasts of the plant into one complete whole, or 

 symplast. It is through these plasmodesmen 

 yy that filtration of the solutions takes place from 



^ ^_ cell to cell under the influence of pressure. The 



leaf parenchyma impermeability of the surface layer of protoplasm 

 cell Torming sugar g^irrounding these fine threads hinders a dis- 

 HsO^^S, sieve tube ordered diffusion of organic substances, thus 

 of phloem ;C, cam- (directing their flow toward the sieve tubes and 

 vLtsei; ' W, root finally to the ceUs where they are utilized, 

 cells. The 'pj^g main consumers of the organic substances 

 dicafe thrflrw'of in the trunk of the tree are the ceUs of the 

 water, the curved cambium, which separate the phloem from the 

 flow'^of sugar' and xylem and Continually fominew tissues. Accord- 

 other organic sub- j^^g ^q Munch, the translocation of the organic 

 double outlines in substances in the tree trunk may be illustrated 

 cells B, s, c, and j^y ^i^^ adjacent scheme (Fig. 110). The paren- 

 iTi Ts m^Ta U^'l^r chyma cells of the leaves B, absorbing carbon 

 Miinch). dioxide from the atmosphere and water from the 



vessels of the wood, elaborate sugars Z. The increase of concen- 

 tration creates osmotic pressure, which forces the sugar solution 

 through the plasmodesmen into the sieve tube S and from there 

 again through the plasmodesmen into the cambial cell C. The 

 cambial cell consumes sugar in the process of growth ; its osmotic 

 concentration falls; and the excess of water is filtered back into 



