428 Lang . — Studies in the Development and 
subjected during development. That the pressure is consider- 
able is indicated by the manner in which the upper surface 
of the sporophyll becomes marked by small depressions 
corresponding to the sporangia of the sporophyll above. 
When this pressure acts upon a sporangium, the form assumed 
at this stage is short and broad, the sporogenous tissue is 
extended parallel to the flat top of the sporangium, and the 
cells of the wall which owe their origin to the primary tapetal 
cells are flattened (Fig. 9). It sometimes happens, however, 
that by the presence of large sporangia around, or by the 
margin of the sporophyll becoming turned down to form 
a sort of rim, the sporangium is almost relieved from pressure 
in the vertical direction. It then assumes the form shown in 
Fig. 10, where the sporogenous tissue forms an elongated 
square prism retaining the general proportions seen in Fig. 7 ; 
the general form of the sporangium is correspondingly 
elongated, and the cells of the wall derived from the primary 
tapetal layer have not the flattened appearance seen in the 
former example. 
The sporangium increases considerably in size without any 
further differentiation becoming apparent. A large mass of 
sporogenous tissue is formed by repeated divisions, and the 
wall increases in thickness (Fig. 12). The thickness of the 
wall is usually greater on the side toward the centre of 
the sorus and the rectangular outline of the upper part of the 
vertical section is due to its thickness at the angles. At the 
sides it consists of four or five layers of cells. At this stage 
the epidermal cells are still thin-walled. The cells adjoining 
the sporogenous mass usually divide by walls parallel to the 
limits of the latter, and the more or less complete double 
layer thus produced is distinguishable even in the mature 
sporangium. This layer, the further development of which 
will be described below, forms a convenient indication of the 
limit between the wall and the sporogenous tissue. 
Until this stage (Fig. 12) the tapetum is not recognizable, 
but shortly after the divisions in the inner layer of the wall 
its origin can be traced. It arises from the sporogenous tissue 
