OF ARTS AND SCIENCES. 89 



three or four iu the frond, or the shigle outer one if there are only two, 

 have begun to disintegrate (Fig. 3, c'). These cortical cells have lost 

 their delinite shape, the walls between them having undergone a pro- 

 cess of absorption. This absorption has not progressed so far, how- 

 ever, that traces of the original outline of the cells cannot still be seen 

 in the stage figured (Fig. 3, d). The contents have also much altered, 

 becoming in the process of disintegration a dark brown color. The 

 absorption and degeneration continue, and finally even the outer cell 

 wall is broken away and the now structureless mass of contents escapes 

 as is shown in Figure 4, c'. Other changes have also taken place. 

 The cells composing the layer next inside the disintegrated cortical 

 cells have enlarged somewhat (Fig. 4, ic), and on looking at the medul- 

 lary cells, which in this region it will be remembered have increased 

 in number, it will be seen that their anterior, or marginal, ends have 

 become thicker, so that the cells are more or less wedge-shaped. The 

 frond has thus become so enlarged, that the cells of what I have called 

 the innermost layer of the cortex are so displaced that, while poste- 

 riorly (B) they retain their connection with the layer of which they 

 are in reality a part, anteriorly (A) they appear to be continuous with 

 the outer layer of the cortical cells of the younger part (Fig. 4, ic'). 

 The cortical cells just posterior to the zone of disintegrated cells de- 

 scribed have begun to grow forwards, and their course being somewhat 

 divergent they turn outwards, finally forming the overlap[)ing ridges 

 (Fig. 2) which give the zoned appearance to the surface of the frond. 

 The cortical cells do not grow on indefinitely, but usually project only 

 the length of a few cells, sometimes, however, growing farther (Fig. 2). 

 As regards the relative stage of development of these zones on various 

 parts of the frond, it may be said that the early stages are always found 

 near the margin of the frond, and, while fully developed ones are 

 sometimes met with close to the edge (Fig. 2), one must look for the 

 oldest farther back. 



The nearest reference that can be found to anything approaching 

 the condition described in this curious overlapping, is in Reiuke's de- 

 scription of Z. parvula, where he says that not infrequently the middle 

 layer was found to be divided irregularly ; * but he does not speak of 

 the formation of zones in this connection, nor does he mention any 

 other of the changes that I have described. 



The part which the overlapping takes in the economy of the frond 

 is a point concerning which only conjectures can be made at best. It 



* Entwickl. Untersuch., v. d. Dictyotaceen, etc., p. 35, Taf. VI. Fig. 4. 



