THE GENUS POLYSIPHONIA. 275 
protoplasmic continuity is maintained throughout the life of the cells. 
Massee (42. p. 198) thinks that the opening becomes closed by the growth of 
a cellulose plate, to the margin of which the primordial utricle is attached, 
He considers, however, that fine protoplasmic threads pass through minute 
holes in the plate, and so maintain the continuity of the protoplasm. Harvey 
Gibson (27. p. 129) has shown, however, that in P. fastigiata, at any rate, 
the canal is closed in the older parts of the plant, Fine granular striæ are 
visible, but maceration shows these to be a fringe of fine threads arising 
along the margin of the plug and quite independent of the protoplasmic 
contents of the canal. 
The structure of the corticate species is similar to that of the ecorticate, 
but in addition to the formation of the primary pericentral siphons in the 
manner described for P. urceolata, there may be an additional formation 
of secondary and tertiary siphons by “ gemmation ” from the primary and 
secondary respectively. When small cells are developed round the peri- 
phery these are produced from the outermost layer of the siphons in a similar 
manner. This is particularly well seen in P. elongata, where protoplasmic 
continuity is clearly visible in a median longitudinal section of the thallus. 
In some of the large corticate forms of which P. elongata is typical the system 
of protoplasmic continuity is more complex than in the ecorticate species. 
In the very early stage there is one connection at each end of the central 
siphon, but as the plant increases in size, four strands are developed round 
the central one in each central siphon, while additional strands also occur in 
the pericentral siphons. If there is more than one row of corticating cells 
at the periphery, the outer layers are formed by gemmation from the inner, 
and protoplasmic connections are maintained. 
The mode of branching is variable throughout the group. It may be 
monopodial, axillary, or pseudo-diehotomous. Rosenvinge (35. p. 222) 
investigated the development of P. violacea. He found that in this axillary 
type the two daughter-cells of the apical cell are unequal in size, giving the 
impression that the smaller cell is eut off from the larger. In P. fastigiata, 
however, very marked pseudo-dichotomy is visible. The apical cell is divided 
hy two oblique walls, and the branches grow out on either side at right 
angles to the inclined septa. 
ATTACHMENT ORGANS. 
The Polysiphonias are usually attached, the only exception being occasional 
floating forms of particular species. A floating form of P. variegata was 
found in the marine lake of Poole Park, and is described on page 308. 
The substratum varies with the species, and may be rocks, stones, shells, 
wood, or other alge. When growing on other algæ the Polysiphonias are 
epiphytic, with the exception of P. fastigiata, which derives food from its host 
Ascophyllum nodosum. The usual type of attachment organ is formed of a 
x 2 
