of locating Sugars in Plant Tissues. 371 
cells naturally are less liable to diffusion than are the contents of more 
superficial cells. 
Similarly, tissues in which intercellular spaces are rare or absent, e. g. 
phloem, are less affected by diffusion than are those in which such spaces 
are large or numerous, e. g. cortex, &c. 
Actual observations of numerous preparations give the impression that, 
after heating for an hour at 98°-ioo° C., diffusion from cells not injured by 
the razor is comparatively small, and that the crystals of osazones formed 
at the surface of the sections and in the medium away from the sections 
themselves are produced mainly from sugars which have escaped from the 
cut cells and have diffused into or have become mechanically mixed with 
the reagent during mounting and subsequent heating. 
In support of this contention it may be stated that in numerous 
instances longitudinal sections of leaf-veins, examined for sugar distribution 
after the leaves had been darkened for suitable periods, have been found to 
show a distinct and fairly continuous gradient in the concentration of 
osazones, which concentration increases towards the proximal end of the 
vein irrespective of accidental variations in the thickness of the long, hand- 
cut sections. 
Often, too, preparations have been examined in which fine sieve-tubes, 
arranged in a single layer, have alone formed a thin portion of the section, 
yet these have been found to be well filled with osazones. 
As far then as intact cells are concerned the distribution of the osazones 
may be held to approximate closely to that of the reacting sugars present 
at the time of cutting the section. 
In other words, positive results may be used fairly safely to locate 
certain sugars in the tissues, while negative results as a rule should not be 
attributed to diffusion of sugar present in the cells at the time of mounting. 
It is well to note, however, that even when sugars are known to be 
present negative results may sometimes be given . 1 
Effects of Glycerine. 
The use of glycerine in the reagent has several advantages. It pene¬ 
trates rapidly, clears up the section, does not evaporate, is a good mounting 
medium, and owing to its viscosity diffusion of sugars, &c., is less rapid in 
it than in water. 
On the other hand, it has some effect upon the reaction with sugars, 
a point which does not appear to have received adequate attention at the 
hands of other botanical workers or critics who have dealt with it. 
Various experiments have been carried out with a view to ascertaining 
1 See below, p. 373. 
