200 



by horizontal walls, the sorus will finally be placed in a groove in the frond (fig. 119 C, 

 comp. Heydrich 1. c. Fig. 3). When rising later it lakes a more superficial position. 



The other specimens arise from disc-shaped ones by very strong enlargment 

 of the under cells of the frond with the exception of the peripheral ones. The 

 figures of Baxters and Heydrich show a great number of large hyaline cells in 

 the interior of the frond, suggesting that the erect cell-rows from which they arose 

 consisted of about 10 cells. Such figures represent, according to my observations, 



eccentric sections in which a greater 

 number of the outwaid bent cell- 

 rows have ])een intersected. Accor- 

 ding to Kylin, the cells of the basal 

 layer remain for a long time un- 

 changed, and differ from the cells 

 of the inner tissue through their 

 small size and rich contents. "Schliess- 

 lich Iritt indessen audi eine Ver- 

 grosserung in den Zellen der Basal- 

 scheibe ein, indem sie sich zwischen 

 die vergrosserten Basalzellen der 

 verticalen Zellfaden einkeilen." This 

 latter assertion is not in accordance 

 with my observations. In specimens 

 attaining only a smaller height, being 

 only cushion-shaped, the cells of the 

 basal layer remain often unchanged, 

 but in typical specimens of the in- 

 llated form they are enlarged at an 

 early period, and there seems to be ordinarily no question of protruding of these 

 cells between those of the second layer. But the cells increasing not only in length 

 but also in breadth, there is no room for all the cells of the basal layer when 

 enlarging their volume, and a number of them must therefore remain unchanged 

 in size. Connected with the growth of the inner cells is the enlargment of the surface 

 of the frond which makes its appearance in the lateral branching of the cell-rows 

 in the periphery of the frond (fig. 120 C). — In the large vesicular cells a number 

 of small chromatophores are easily visible; in some cases these cells were poor in 

 cell-contents, in others they contained small starch grains. 



The simultaneous occurrence of the two forms of the species on the same 

 leaf of Zostera is very curious. As a rule, the specimens growing on the faces of 

 the leaves are disc-shaped or low cushion-shaped, while those placed on the margins 

 are inflated. Cushion-shaped specimens may, however, be found on the margins 

 and inflated on the faces, thus the two forms of specimens may occur side by side 

 apparently under equal external conditions; this may perhaps be caused by a different 



Fif{. 119. 



Rltodoilermis Georr/ii A. basal layer seen from below, showing 

 the border and lateral fusions. B, marginal part of Irond in 

 vertical section, showing a young liair. ('.. vertical section of 

 disc-shaped frond showing a sorus sunk in a groove. 350:1. 



