278 DR. LILY BATTEN ON 
siphons ; the actual number is, however, subject to slight variation, These 
rhizoids are terminal in origin, and not lateral like the subsequent ones, As 
growth proceeds and the thallus branches, any pericentral siphon of a 
procumbent branch may develop a rhizoid, and thus afford additional 
strength for the attachment of a colony. Ultimately then, the colony is 
attached by a large number of rhizoids which arise from the main axis and 
low branches, but there is no aggregation into any special form. 
In this type of attachment the tips of the rhizoids may ramify among the 
algæ and debris, and not become expanded as in the figure of P. fwtidissima 
(Pl. 25. fig. 64), or they may develop expanded discs as in P. urceolata 
itself, or become swollen as haustorial organs, as in P. fastigiata. 
2. P. nigrescens. 
The main principle of the attachment is similar to that of P. urceolata, but 
at the extreme base of the thallus a row of corticating cells frequently occurs. 
This appears to be correlated with the closer development of rhizoids, and 
the fact that an aggregation is possible whereby the discs of the colony 
adhere mechanically to each other, making an uninterrupted surface at the 
base of the attachment. This mechanical coherence of the discs is not so 
marked in the ecorticate species of Polysiphonia, and it appears that the 
phenomenon is made possible by the presence of the corticating cells and 
consequent closer development of the rhizoids, and is also facilitated by the 
presence of a greater number of pericentral siphons than is present in the 
group typified by P. urceolata. It appears to be the first step towards 
the aggregation of the individual dises into a single large attachment organ, 
such as is seen in P. elongata, but there is no sign of the coalescence of the 
upper parts of the rhizoids. 
3. P. fruticulosa. 
P. fruticulosa is corticate throughout, and rhizoids are formed in large 
numbers from the corticating cells. There is, however, a special adaptation 
which is of interest. The young plant is attached at the base by a number 
of terminal rhizoids. When branching takes place, certain of the lower 
branches are procumbent for a short distance, and themselves branch alter- 
nately as they creep along the substratum. Of these secondary branches, the 
upper ones continue growth in an upward direction, while those on the under 
side of the thallus become stunted and short. The corticating cells of these 
stunted branches send out large numbers of rhizoids, and the whole branch 
appears to be devoted to the function of attachment (Pl. 25. fig. 78). The 
tip of the branch frequently bends round the thallus of any small alga which 
it may encounter, and thus affords additional strength. A few rhizoids may 
develop from the procumbent parts of the original branches, but the principal 
means of attachment is the modified branchlet, There is no mechanical 
