136 Grubb .— The Attachments of Porphyra umbilicalis. 
and to some extent explains the tenacious adherence to the rocks that is 
exhibited by the discs of Porphyra. 
As the filaments forming the disc wind downwards towards the base, 
many of them turn outwards and form the outer tissue of the structure, the 
free tips being enclosed in thick gelatinous walls. In older discs cells are 
sometimes cut off in great numbers from these tips, each with a single 
nucleus; these cells may be aggregated together and form a compact 
parenchymatous layer on the outer surface (Text-fig. 7, B). The interest of 
this differentiation of tissue lies in the fact that this parenchymatous tissue 
is capable, by cell division, of creeping along the surface of the ground and 
of putting up new fronds. For example, in Text-fig. 4 a large central disc 
has given rise laterally to two creeping discs, while other and younger 
fronds are growing out from the main disc. 
Text-fig. 4. Outline drawing of a proliferating disc of Porphyra umbilicalis. The actual 
attaching surface (a) of the central disc is shown giving rise on either side to lateral attaching surfaces. 
From the upper surface of the main disc four young fronds have arisen (b) as well as the main frond. 
Total length of the attaching surface, 5-5 mm. 
That vegetative reproduction takes place in this way by a creeping and 
proliferating disc has been shown by Hus ( 7 ) for an epiphytic species of 
Porphyra — P. naidum , And.—from the Pacific coast of North America. 
This species has a parenchymatous or ‘ prothallial ’ base from which new 
fronds are developed ; the base consists of small disc-shaped outgrowths 
which are one cell in thickness and gradually creep over the blades 
of Zoster a ; here and there the tissue becomes several layers thick, and 
from the external layer short blunt protuberances are put out which 
develop into fronds. I have also noted something similar in Enteromorpha 
compressa , for in a slide lent me by Dr. Delf two young narrow vertical 
fronds of Enteromorpha are clearly connected by a horizontal creeping 
parenchymatous base, one layer of cells in thickness (Text-fig. 5). This 
type of vegetative reproduction by a proliferating base seems to be present in 
