VIII] OR CELL-AGGREGATES 407 



section). The effect, as seen in section, is to cut off on each side 

 a characteristically flattened cell, oblong as seen in section, still 

 leaving a triangular (or strictly speaking, a tetrahedral) one in 

 the centre. The former cells, which constitute no specific structure 

 or perform no specific physiological function, but which merely 

 represent certain directions in space towards which the whole 

 system of partitioning has gradually led, are called by botanists 

 the "tapetum." The active growing tetrahedral cell which lies 

 between them, and from which' in a sense every other cell in the 

 system has been either directly or indirectly segmented off, still 

 manifests, as it were, its vigour and activity, and now, by 

 internal subdivision, becomes the mother-cell of the spores. 



In all these cases, for simplicity's sake, we have merely con- 

 sidered the appearances presented in a single, longitudinal, plane 

 of optical section. But it is not difficult to interpret from these 

 appearances what would be seen in another plane, for instance 

 in a transverse section. In our first example, for instance, that 

 of the developing embryo of Sphagnum (Fig. 183), we can see that, 

 at appropriate levels, the cells of the original cylindrical row have 

 divided into transverse rows of four, and then of eight cells. We 

 may be sure that the four cells represent, approximately, quadrants 

 of a cyhndrical disc, the four cells, as usual, not meeting in a point, 

 but intercepted by a small intermediate partition. Again, where 

 we have a plate of eight cells, we may well imagine that the eight 

 octants are arranged in what we have found to be the way 

 naturally resulting from the division of four quadrants, that is to 

 say into alternately triangular and quadrangular portions ; and 

 this is found by means of sections to be the case. The accompany- 

 ing figure is precisely comparable to our previous diagrams of the 

 arrangement of an aggregate of eight cells in a dividing disc, save 

 only that, in two cases, the cells have already undergone a further 

 subdivision. 



It follows in like manner, that in a host of cases we meet with 

 this characteristic figure, in one or other of its possible, and 

 strictly limited, variations, — in the cross sections of growing 

 embryonic structures, just as we have already seen that it appears 

 in a host of cases where the entire system (or a portion of its 



