VIII] OR CELL-AGGJIEGATES 641 



confine ourselves to the appearances as seen in section). The effect 

 , is to cut off on each side of the apical cell a characteristically 

 flattened cell, oblong as seen in section, still leaving a triangular 

 (or strictly speaking, a tetrahedral) one in the centre. The oblong 

 cells, which constitute no specific structure and perform no specific 

 physiological function*, but which merely represent certain direc- 

 tions 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 its vigour and activity, 

 and becomes, by internal subdivision, 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 oi 

 the developing embryo of Sphagnum (Fig. 281 c, d), we see that, 

 at appropriate levels, the cells of the original cyhndrical 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 cylindrical 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 figure is precisely com- 

 parable to our previous diagrams of the arrangement of eight pells m 

 a dividing disc, save only that, in two cases, the cells have already 

 undergone a further subdivision. 



It follows that we are apt to meet with this characteristic figure, in 

 one or other of its possible and strictly limited variations, in the 

 cross-sections of many growing structures, just as we have already 



* This is not to say that Nature makes no use of the tapetal cells. In the end 

 they break down and contribute to the growth of the spore-mother-cell: very 

 much as the "superfluous" eggs in a fly's ovary contribute yolk-material to the 

 developing ovum. 



