THE GENUS POLYSIPHONIA. 279 
cohesion of the upper parts of the rhizoids, but the expanded bases may fit in 
together, though I have never found a definite plate such as one sees in 
P. nigrescens. 
4. P. elongata. 
P. elongata is typical of the most complex form of attachment which is 
exhibited in the genus. A median longitudinal section shows that the 
siphons themselves, as well as the corticating cells, take part in the formation 
of the large dise by means of which the plant is attached (Pl. 24. fig. 47). 
The central and pericentral siphons, together with a large number of corti- 
cating cells, are elongated longitudinally into rhizoids, and as elongation takes 
place, the cells and siphons at the base spread out, so that a bigger surface is 
available for attachment. The outer rhizoids develop dises comparable to 
those of P. urceolata, but the discs of tie inner rhizoids are smaller on 
account of crowding. The upper parts of the latter mechanically coalesce to 
form a pseudo-tissue, and it is only on the outside at the extreme base of the 
attachment that the separate walls of individual rhizoids may be discerned. 
With this form of attachment it is only the base of the main axis which is 
involved, so that none of the branches are procumbent, and consequently 
plants with an individual axis are characteristic, instead of a colony as in 
P. nigrescens. 
Taking these four species as typical of the groups which they represent, it 
is apparent, therefore, that the more complex type of attachment organ can 
be correlated with the development of cortication in the genus. In the 
simple ecorticate forms the attachment organ consists of rhizoids which are 
developed by the proliferation of the siphons themselves. As cortication 
begins to be visible, the smaller outer cells form the attachment, and there 
are signs of an aggregation into disc-like form. When the whole plant is 
corticate, the attachment is either formed by means of a modified branch as 
in P. fruticulosa, or the siphons and corticating cells take part in the forma- 
tion of a dise as in J’. elongata. Intermediate forms occur, as for example 
P. Brodiei, where the original dise is comparable to P. elongata, but the 
procumbent branches afford additional mechanical strength by sending out 
rhizoids at their bases. 
REPRODUCTION. 
Tetraspores. 
Tetraspores are borne in the ramuli, and may either occupy the tip of the 
ramulus, as is usual in P. urceolata, or may be intercalary in development as 
in P. Brodiei. As Massee (42. p. 198 et seg.) has pointed out, the tetra- 
sporangium originates in the same way as the pericentral siphons, and in 
P. fastigiata occupies a space equal to that occupied by two siphons. Two 
daughter-cells are developed on the outside by gemmation from the tetra- 
sporangium (Pl. 22. fig. 9), which is in communication with the axial siphon 
