Delf . — The Attaching Discs of the Ulvaceae . 407 
saprophytic nature of the fronds. The host plant had been completely 
encircled by the developing disc, and the thick walls of the pericentral cells 
were pierced and riddled with the small pointed ends of the filaments, which 
usually contained more than one nucleus. In many of the cells the tips of 
the filaments had evidently expanded, forming a delicately vacuolated lining 
of cytoplasm almost filling the cell, and containing numerous and character- 
istic embedded nuclei (PL XLV, Figs. 9, 10). At the periphery of the host 
may be seen the appressed tips of large and small filaments, just as in the 
disc already described ; both these kinds of filaments were found in the 
process of boring their way through the solid wall, and this is probably 
achieved, as in the Fungi, by the secretion of some cytohydrolithic ferment. 
It is impossible for me to say whether the host plant was living at the 
time of fixing, but I have myself often observed both Ulva latissima and 
Ulva lactuca growing attached to the frond of some other living seaweed, 
and it seems probable, therefore, that this plant is at least a facultative 
parasite. Since, however, it is known that both species of Ulva grow 
luxuriantly in the presence of the nitrogenous products of decay, it is 
probable that the saprophytic habit could also be readily adopted. 
Theoretical Considerations. 
It is well known that the different species of Ulva which commonly 
occur on our coasts are especially abundant in the brackish water of estuarine 
localities, and also on the open shore in the neighbourhood of towns. In 
some cases the fronds grow so luxuriantly as to be cast up in masses by the 
tide, and the offensive smell of the sulphuretted hydrogen which is formed 
as these plants decay is sometimes a serious nuisance to the inhabitants. 
Before, however, this can be effectively remedied, much more must be known 
of the life-history and habits of the plant, as has already been shown in 
detail by Mr. A. D. Cotton in his suggestive paper on the subject. 1 
From the above account it appears that there is probably a very 
effective means of reproduction in the formation of these adhesive discs, 
which are undoubtedly very tenacious of life, can probably live through the 
winter independently of the thallus (which is described by Harvey and others 
as annual), and can bear new fronds by the upgrowth of the filamentous 
tubes of the disc. It would therefore be necessary to adopt some measure 
to destroy the vitality of these in order to check their spread effectively. 
From the point of view of affinity, the multinucleate character of the 
filaments of the disc in Ulva serves to emphasize the isolation of the whole 
group of Ulvales, and to remove it from any near relationship with the 
Ulothricales. On the other hand, the simple segmentation of the tips of 
the filaments has no resemblance to the method of wall formation in 
1 Cotton, A. D. : On the Growth of Ulva latissima in Water polluted by Sewage. Kew 
Bulletin, 1910. 
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