284 BOTANICAL GAZETTE [OCTOBER 
of the sporophyll. Dorsal to the strand lie transfusion elements 
distinguishable by their short length and reticulated walls. The 
tracheids of the bundles in their upward course become ever shorter, 
and the spiral or scalariform sculpture of the walls is gradually 
transformed into the reticulations which characterize the trans- 
fusion elements in which the bundle ends. The transfusion cells 
in turn pass by gradual transition into the mechanical elements, 
which are responsible for the opening of the sporangium at the 
time of maturity of the spores. It will be seen that the mechanical 
cells are not confined to the region of the sporangium wall, but 
pass up into the median horn, which terminates the sporophyll. 
In some cases the former actually more or less completely surround 
the secretory space at the apex. In the mature sporangium the 
mechanical elements extend to the sporangial cavity, being ordi- 
narily separated from it only by a membrane formed from the 
disorganized tapetum. The mechanical cells form a continuous 
jacket, and are only interrupted in the longitudinal inner line 
corresponding to the stomium (not shown). ‘The figure clearly 
shows that the mechanical elements are not present in the epi- 
dermis. The diagrammatic plane of section of the sporangium and 
sporophyll is necessarily somewhat to one side of the median line 
of the termination of the corresponding fibrovascular strand, since 
the latter continues into and is merged in transfusion elements 
and mechanical cells in the region of the septum between the two 
spore sacs. For that reason the upward end of the strand does not 
appear in the field of view. The explanation of the situation Is 
furnished by reference to the horizontal section shown in fig. 3- 
It is clear from the descriptions given in the foregoing para- 
graphs that the fibrovascular bundles of the sporophylls in Ginkgo 
end in transfusion tissue, which in turn passes, often almost imper- 
ceptibly (fig. 4), into the mechanical elements of the sporangial 
wall. It thus is obvious that in this genus the opening mechanism 
of the sporangium is an adjunct if not a derivative of the fibro- 
vascular system. This situation recalls the interesting observa- 
tions made by Ortver over a decade ago in the case of an isolated 
3 OLIVER, F. W., Ona eee! sporangium from the Stephanian of Grand’ Croix. 
New Phytol. 1: 60-67. pl. 1 ‘ 
