88 PROCEEDINGS OF THE AMERICAN ACADEMY 



entire ; but it soon becomes split, though to a less degree than in some 

 species of Zonaria. The splitting takes place by a process very like 

 that described by Reinke in the case of Taonia atomaria. Usually a 

 small number of the marginal cells die, and their contents disintegrate, 

 forming a brownish structureless mass. Since the marginal cells on 

 either side continue to grow, there is formed a cleft, which gradually 

 grows deeper and deeper, finally forming a split which becomes visible 

 to the naked eye. 



It was in a vertical longitudinal section of the frond which I had cut 

 to study the apical growth, that a peculiar appearance like that shown 

 in Figure 2 was seen. This occurred to me at once as strange, and 

 unlike anything I had seen before. No reference to any such condition 

 could be found in the literature on the subject, and, unless lam greatly 

 mistaken, it has never been mentioned. This singular condition when 

 fully formed may be briefly described as an overlapping forward of 

 the cortical cells over the frond. That to this overlapping the appear- 

 ance of transverse zones is due, becomes evident on viewing it with a 

 low power from above, when the lines of the projecting cortical cells 

 may be seen at more or less regular intervals. These zones should 

 not be confounded with the comparatively fine and indistinct lines 

 which run parallel to them, and are only to be seen on viewing the 

 frond obliquely. The latter markings are apparently due to slight 

 undulations of the cortical cells, while the zones referred to are much 

 more complicated in structure. In a vertical section of the zones in 

 their fully formed condition the most prominent feature are the out- 

 growths on both sides of one or two layers of the cortical cells. But 

 besides this, it is seen that in the region of the zonal growth the me- 

 dullary cell has become divided into three or four parts horizontally, 

 and that the whole frond in that place is somewhat thickened (Fig. 2). 

 The earliest stage in the development of these zones that I was able to 

 find in the material at hand was young enough to give pretty clear 

 evidence as to their origin. In this stage the medullary cells in the 

 region of the zone about to be formed became divided into three or 

 four cells by the formation of longitudinal walls parallel to the surface 

 of the frond (Fig. 3, n/). The medullary cells which undergo division 

 in this way form usually a single row parallel to the margin of the 

 frond, but occasionally, instead of a single, they consist of a double row 

 (in Figs. 3 and 4 the direction of the margin of the frond is indicated 

 by the letter A). At the same stage the cortical cells (Fig. 3, c) 

 in the immediate neighborhood of the developing zone can be seen to 

 have undergone a change. The two outermost layers if there are 



