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 phloém. 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 anesthetic or by some more 
violent and lethal agent. 
