138 Fritsch.— The Morphology and Ecology of an Extreme 
responsible for the diverse views put forward by different authorities as to 
the nature of the chlorophyll-apparatus. The chloroplasts of the ordinary 
species of Zygnema are stated to divide at a time when the ingrowth of the 
septum between the daughter-cells is almost completed (de Bary, ’ 58 , p. 11 ; 
Wille, ’ 90 , p. 17), the young cell thus having two chromatophores except 
during early stages of division. In this respect even the Hindhead form 
differs markedly, since doubling of the chloroplasts is delayed until some 
considerable time after the development of the septum between the 
daughter-cells or, frequently, until the next division occurs. 
The nucleus of the mature cell of the Hindhead form is situated mid- 
way between the two chloroplasts and is oval in shape, the long axis being 
placed parallel to the longitudinal walls of the cells 1 (Fig. 3, c). In the 
younger cells, with a single chloroplast, it appears to be rounded and 
apposed to the latter (Fig. 3, D, E) (cf. also West and Starkey, T 5 , p. 197). 
A characteristic feature of the cell-contents of Zygnema ericetorum is 
the presence in the sap of a purple pigment, phycoporphyrin, which has 
been the subject of a detailed study by Lagerheim (’ 95 ). The occurrence 
of this pigment is, however, somewhat variable. Hassall (’ 45 , p. 175) refers 
to bright green filaments observed only in aquatic habitats, and purple 
filaments characteristic of the form spreading over swampy heaths. The 
aquatic form collected at Frensham, however, had purple sap, whilst part 
of the material sent to me by Mr. Tabor had little or none, although 
growing close to, but not intermingled with, other purple-coloured masses, 
the two apparently being subjected to identical conditions. 
Lagerheim (’ 95 , p. 23) has pointed out that it is only the upper portion 
of a patch, i. e. the part which is strongly illuminated, that develops the 
purple pigment, and the same observation has been made on the Hindhead 
form. 2 This would indicate that the pigment is, at least in part, a pro- 
tection against strong illumination, and the following appears to lend further 
support to this view. At one point the Hindhead form is becoming over- 
grown by bright green filaments of a Ulotrichaceous Alga, that seems to 
correspond to Hormidium flaccidum , A. Br. (Fritsch and Salisbury, T 5 , 
p. 1 31). An examination of the underlying filaments of the Zygnema 
shows that the chloroplasts have shifted from the middle to the upper sides 
of the cells, so that these appear green, in contrast to the purple-coloured 
under sides. It seems that, as a result of the screen formed by the 
Hormidium , the chloroplasts of the underlying Alga do not receive 
sufficient light, a disadvantage remedied by their moving to the upper sides 
of the cells. A patch of the typical purple-coloured Hindhead form was 
1 I am much indebted to my colleague. Dr. Salisbury, for kindly microtoming material of 
the Alga. 
2 The green filaments from the under side of the patch are excellent material for demonstrating 
the two massive chloroplasts of the mature cell. 
