58 STRUCTURAL BOTANY 



guished by the unicellular origin of their sporangia. 

 A single cell, then, grows out from the surface of the 

 placenta and soon begins to divide. One or two basal 

 cells are often cut off, to begin with, by transverse walls, 

 but they are of no great importance. The terminal cell 

 next undergoes division by inclined walls (see Fig. 28, 6r), 

 three of which are formed in succession, inclined to each 

 other at an angle of 120, as seen from above. In 

 side view, as shown from the figure, only two of these 

 walls can be seen, and they join each other at an acute 

 angle. The result of these three divisions is to carve 

 out a three-sided pyramidal cell with a free base, quite 

 like the apical cell which we have already described in 

 the stem and root. The next wall formed runs parallel 

 to the free base of this pyramidal cell, so now we have a 

 central cell surrounded on all four sides by the segments 

 which have been cut off from it. The segments undergo 

 a great many more divisions, and form the wall of the 

 capsule, which remains only one cell thick, as all the 

 cell-divisions are at right angles to its surface. The 

 lateral segments also cut off cells below, which go to build 

 up the stalk. 



In the mean time the pyramidal central cell has itself 

 divided by walls parallel to its four sides, so that it is 

 now surrounded by an inner layer of cells separating it 

 from the wall of the capsule. These intermediate cells 

 undergo further divisions in various directions and form 

 the tapetum, the ultimate destiny of which is to afford 

 food material to the developing spores (see Fig. 28, F t iw). 



The central cell which remains is the essential part 

 of the whole structure, for this is the archesporium, from 

 which the spores themselves are produced (see Fig. 28, 

 F, c). We see, then, that in this case the archesporium 



