32 
HISTOLOGY OF VEGETABLES. 
of the nature of the grains, the Doctor was enabled to 
detect the imposition. 
Could it be proved that a direct relation exists 
between the size of the starch granules and their 
nutritive quality, we should, of course, possess a ready 
method of determining such quality in any given speci- 
and, on this principle, Tons les mois would he 
men 
FIG. 20 . 
considered as much more nutritious than the potato, 
and the potato than the rice starch. 
Starch possesses the property of polarizing light ; each 
grain shows a black cross surrounded by coloured rings, 
changing to a white cross, with the complimental colours 
of lower intensity when the analysing prism is revolved. 
In the euphorbiaceous plants, the starch granules 
have a peculiar form and situa- 
tion ; they are found in the in- 
terior of the laticiferous vessels, 
floating in the latex or milky 
juice. Fig. 20 b, and their shape 
is that of an elongated dumb- 
bell, or two-headed club, as 
shown at a . They are easily ex- 
hibited by wounding the plant, 
and placing a drop of the milky 
juice under the microscope, and 
these singular granules are proved to be really starch, 
by the addition of tincture of iodine, which gives them 
a deep blue tinge. 
Before leaving the subject of starch, allusion may be 
a, club-shaped starch grains 
of Euphorbia splendens ; b, three 
of the laticiferous vessels with 
starch grains in situ. 
