OP ARTS AND SCIENCES. 



53 



of the Polypi phonia to any great extent, it will be seen that the pe- 

 ripheral cells of the external portion of the Ciioreocolax frond present 

 an appearance different from those in the interior. They have become 

 arranged in a regular layer one cell deep over the entire surface of 

 the frond, covering the more or less promiscuous mass of cells be- 

 neath (Fig. 4). The internal cells divide and grow to some extent, but 

 it is fiom the outer, regular layer of cells that the larger part of the 

 exterior portion of the frond is to be developed. The growth inside 

 of the host plant also helps in the formation of this part of the frond, 

 but it is only for a short tune that it can be distinguished from the 

 growth of the peripheral cells just mentioned. As soon as the cells 

 which arise from the inner filaments make their way between the cells 

 of the Polysiphonia to the outside, they become arranged in this reg- 

 ular order and blend with the rest of the frond, becoming indistinouish- 

 able from it. Cases have been seen where, owing to irregular growth, 

 they did not unite ; and then, instead of one laige protuberance, there 

 were many smaller swellings closely bunched together. The surface 

 of the young frond, at first almost a plane, becomes rapidly convex by 

 the more active growth of the cells in the centre of the frond. Finally, 

 the hemispherical or almost spherical mass is formed in which the 

 fruit is later borne. 



The ordinary method of growth of the frond in distinction to the 

 manner of development in its earlier stages is now to be considered. 

 It is essentially the same after the condition is reached where the 

 peripheral cells are arranged in a distinct layer. Before that time 

 the growth is irregular and unequal. Taking a single peripheral cell 

 and following its growth throughout, we find the method to be as fol- 

 lows. First, the cell is divided into two parts by the formation of a 

 transverse wall. The lowest half of the cell does not divide again, but 

 merely increases in size. The upper cell, on the other hand, is soou 

 divided in two by the foimation of a vertical wall, and these two cells 

 ultimately become four by the division of each into two, in a vertical 

 direction at right angles to the first vertical division (Fig. 6). The 

 four cells thus formed repeat the process of division first described, 

 and by this means the frond is enlarged in all three dimensions. The 

 number of cells into which the outer row of cells may divide vertically 

 is not necessarily four. There may be only three, or sometimes as 

 many as five cells so formed, but the important point is that they are 

 equally distributed, so that, besides extending the frond vertically, they 

 increase it almost equally in both of the other directions of space. 

 Other irregularities are also noticeable ; sometimes the transverse 



