68 PLANT LIFE 



strand, and some have the transverse walls 

 which separate the elongated cells of a row 

 perforated by small pores. These are the 

 sieve tubes, and much of the various food 

 substances which reach the vascular strands 

 passes through them. But it is probable that 

 such an easily diffusible substance as sugar 

 passes as well through tracts of other elon- 

 gated, but not so obviously perforated, cells 

 of the phloem. Be this as it may, it is 

 largely through the vascular strands that the 

 sugars of the plant are carried away from 

 the regions where they are present in excess 

 to other regions where they are relatively 

 deficient. This occurs whether the deficiency 

 arises through the sugar being directly used 

 up in the chemical operations of the cells, 

 or whether the special conditions of the local 

 deposition of food reserves are such as to 

 produce a diffusion gradient, that is a steady 

 flow within the plant from a place of high to 

 one of lower concentration. It is well to 

 emphasise the limitation thus expressed in 

 the last sentence, for however readily sub- 

 stances may travel from one plant cell to 

 another, it is a very different thing if one 

 endeavours to get them to diffuse out of the 

 region of the living cells into a mass of sur- 

 rounding water, for example. Such attempts 

 commonly do not succeed unless the cell 

 protoplasm be first modified, as, for example, 

 by means of an anaesthetic or by some more 

 violent and lethal agent. 



