47 2 Carter. — The Cytology of the Cladophoraceae. 
but frequently penetrates into the interior and ramifies in all directions 
(Figs, i, 2, 4, 6 , and 7 ; Text-fig. i, A, F, and G, Text-fig. 2, B). 
Again, the extent of this depends on the individual cell. In the older 
segments of Cladophora and the large segments of Chaetomorpha Linum , 
(Muell.) Kutz., the entire chloroplast is confined to a very thin parietal 
lining layer, and the interior of the cell is occupied by a large vacuole, being 
destitute of both protoplasm and chloroplast (Figs. 5 and 8 ; Text-fig. 2, c). 
In the younger segments of Cladophora , however, and the rather smaller 
segments of Chaetomorpha gracilis, Kutz., and some forms of Rhizoclonium 
hieroglyphicum , (Kutz) Stockm., which are much richer in cell-contents, the 
interior of the cell is often traversed by irregular ramifying strands of 
chloroplast (Figs. 1, 2, 4, 6, and 7 ; Text-fig. 1, A, F, and G, Text-fig. 2, B). 
The pyrenoids are very numerous, particularly in Chaetomorpha and Cla- 
dophora , and are to be found in every part of the chloroplast (Figs. 1-8 ; 
Text-figs. 1 and 2). It was noticed in Rhizoclonmm hieroglyphicum that 
the pyrenoids in the internal strands of chloroplast were much larger in 
size than those occurring in the thin parietal lining layer, possibly because 
they can develop far more freely in this position (Text-fig. i,f). 
When the interior of the segment is filled by these ramifying strands of 
chloroplast it is often very difficult to distinguish between the chloroplast 
and the colourless cytoplasm. Of course, when the external parietal sheath 
of chloroplast is reticulated it is quite easy to demonstrate the colourless 
cytoplasm filling in the perforations in the chloroplast (Fig. 3). Where the 
chloroplast is almost continuous over the whole cell-wall, however, colourless 
cytoplasm is often apparently non-existent. In sections, also, it often 
appears that every available strand of cytoplasm is occupied by chloroplast 
and pyrenoids (Figs. 1 and 6). It is sometimes possible, however, to dis- 
tinguish extremely delicate, nearly colourless, granular strands, destitute of 
pyrenoids and usually of nuclei also, which doubtless represent the colourless 
cytoplasm (Text-fig. 2, b) ; but very frequently the chloroplast is so extensive, 
and penetrates both the lining layer of cytoplasm and the internal strands 
traversing the lumen of the cell so completely, that the colourless proto- 
plasm is reduced to an extremely thin film enveloping the chloroplast — so 
thin that it cannot be detected. 
The relation of the nuclei to the chloroplast is rather unusual in these 
three genera. In other algae where there is a definite chromatophore, the 
nucleus is usually confined to the colourless cytoplasm, but in Cladophora 
and Chaetomorpha particularly the nuclei are nearly always more or less 
completely immersed in the chloroplast. They are scattered throughout 
both the parietal lining sheath of chloroplast and also in the internal strands 
traversing the cell-cavity (Figs. 1, 2, 4-8 ; Text-figs. 1 and 2). In the parietal 
layer no definite colourless cytoplasmic sheath can be detected round the 
nucleus, and in the interior of the cell they usually occur in such close 
