176 
Rendle . — On the Vesicular 
of callus on the walls and the general vigour of the life of the 
plant or its members. The resemblance to sieve-tubes is 
simply structural ; and it may be noticed that these members 
of the vascular bundles of the onion are by no means few or 
feebly developed, so as to necessitate any additional means 
of transport for nutrient substances. These vesicular cells 
are evidently merely rows of excretory sacs which begin to 
be sealed up by callus very early, the smaller amount of 
callus in the green leaves corresponding with the less evident 
pitting found there. This view of their function is confirmed 
by the following observation. In the germination of a resting 
onion which had produced a vigorous shoot a foot high, and 
in which the formerly succulent leaves were being used up 
from the outside inwards, the vesicular cells in the now quite 
membranous scales were still crowded with the resinous latex, 
and became marked out as pink lines in the leaf on placing 
pieces in dilute alcanna-tincture. The transverse septa were 
still callosed, while the nuclei had almost completely broken 
down. Thus, at a time when the tissue had been so drawn 
upon to supply the growing shoot, as to reduce the leaf to the 
consistency of thin paper and to render it quite transparent, 
these laticiferous cells were still full of what must I think 
be therefore regarded as a purely excretory substance, one, 
that is, of no further use to the plant as a food-stuff. No 
other excretory structures are found in the tissues of the stem 
and leaves of the onion. Doubtless the resinous latex is of 
some use in the life of the plant, perhaps making it unpalatable 
to certain animals, or keeping them off by the peculiar smell. 
This smell is due to some substance which gives rise to allyl- 
sulphide when an onion is analysed, and which is probably 
contained in the latex, since a silver coin is slightly blackened 
thereby; but, though I have tried a great number of micro- 
chemical tests, I have never succeeded in demonstrating that 
this compound is localised. It is probably only present in 
small quantities, the detection of which is made practically 
impossible by the presence of so much resinous substance, for 
resins greatly interfere with the tests. 
