Latex arid its Functions . 207 
at this point, due to the pressure upon them of these altering 
cells. 
About one hundred laticiferous plants have been tested 
regarding the flow of latex from the cut petiolar base, and in 
only one other plant, Plumiera acutifolia, has a case been 
found at all as striking as that of Hevea . 
Plumiera acutifolia. The flow of latex from the cut petiole 
takes place precisely as in Hevea. Unlike it, however, a 
microscopic examination reveals no special structural changes 
in the base of the petiole on the leaf attaining maturity. 
There is no formation of £ stone ’ cells or deposition of crystals, 
the only difference being that the laticiferous tubes are less 
marked in the adult than in the young leaf. 
Plumiera rubra behaves similarly, but P. obtusa does not 
exhibit this peculiarity ; latex issues from both surfaces when 
the base of the petiole is cut across. 
Contrasting the two cases of Hevea and Plumiera it looks 
as if the interference with the flow of latex, at the base of the 
petiole, may be brought about, not by any special form of 
cellular tissue closing the tubes, but rather by the pressure 
of the ordinary parenchyma at a region destined to be that 
of the absciss-layer. 
Microscopic examinations have failed to show any definite 
obliteration of the cavities of the laticiferous vessels, crossing 
from the leaf to the stem. They seem, however, to be more 
crushed here than elsewhere in the petiole. 
The severing experiments are difficult of explanation on 
any other supposition than that of the obliteration of the 
cavities of the tubes, unless it be that the block occurs in 
the tubes themselves, as by a coagulum forming in the latex, 
or by an ingrowth of the walls of the tubes. 
Since this phenomenon is so exceptional amongst laticiferous 
plants, it may perhaps be looked upon as merely accidental, 
and an extreme case of the general tendency for laticiferous 
tubes to be somewhat crushed in mature tissues. 
Nevertheless, it seems to me that this peculiarity affords 
to some extent an argument against the view that the latici- 
