230 BOTANICAL GAZETTE [APRIL 
400 or 500 per cent., is accomplished by the growth of the indi- 
vidual cells. * 
The formation of the air cavities is interesting, since it is 
comparable in some respects to the differentiation of trabecular 
and sporogenous tissue in the sporangium. Ina leaf such as is 
represented in cross section in fig. g, there is yet no sign of the 
air chambers, Increase of diameter is actively going on and 
the whole leaf is still meristematic. In the leaf shown in jig. 
ro the position of the future air chambers is indicated by four 
symmetrically placed groups of cells which have lost most of 
their contents. The peripheral cells of the leaf, the central 
cells, and four radiating bands which appear in cross section as 
spokes arranged in the form of the sign + continue to grow and 
are distinguishable by their larger more densely filled cells. 
Stained with Delafield’s haematoxylin and erythrosin these 
cells show deep red cytoplasmic contents and large nuclei in 
which the red staining predominates; while in the areas which 
are to become air chambers the cytoplasmic contents have 
almost entirely disappeared, but the nuclei still retaining their 
chromatin stain intensely with haematoxylin. When only a 
nuclear stain is employed, such as iron-alum-haematoxylin, the 
four non-protoplasmic areas are rendered very prominent by 
their black nuclei. Longitudinal sections show that the regions 
which are thus sharply distinct in cross section run lengthwise of 
the leaf in unbroken bands from just above the ligule nearly to 
the apex, and there are as yet no air cavities. 
The air chambers are formed lysigenously. The growing 
tissues generate a tension in the empty cells, and as a result 
these are ruptured irregularly, and small cavities appear, sepa- 
rated by diaphragms or plates of cells extending across from 
the central to the peripheral growing regions. As the leaf elon- 
gates, the air cavities increase in size, while the diaphragms drawn 
farther and farther apart lose their protoplasm to the surrounding 
cells. When once this splitting into diaphragms and cavities 
has occurred, it is not repeated ; there remains no meristem in 
which they may be generated. Occasionally single diaphragms 
