130 



' 



Fig. 138. Peyssonnolid 

 simulans n.sp. Straight 

 running filaments of the 

 hypothallus. (160:1). 



direction over the substratum (Fig. 138), 2. Cruoriella with a 

 hypothallus of curved filaments running in little fan-shaped or 



broad-lanceolate groups over the substra- 

 tum (Fig. 139) and 3. Ethelia with no 

 hypothallus but a mesothallus that gives 

 off branches both downward and upward. 

 This division proved very useful while 

 working out the Peyssonnelia of Dr. BOR- 

 GESEN. In his collection species of Peys- 

 sonnelia and Cruoriella are numerous but 

 the subgenus Ethelia is wanting, it has 

 till now been only found in the East- 

 Indian seas and in the Mediterranean. P. 

 squamaria, the well-known inhabitant of 

 the last-named sea, has a true mesothal- 

 lus though the perithallus inferior is re- 

 duced to only one layer of cells. 



It is a well-known fact that alga? 

 have a great variability and in how far 

 the Peyssonnelia are subject to this general law, is still an open 

 question, for this group of plants has relatively been little studied. 

 We know that the circular or lobed fronds can be membranaceous 

 (P. rubra), coriaceous (P. squamaria), or calcareous and hard as 

 stone (P. polytnorpha), but there are also 

 other forms not entirely calcareous neither 

 coriaceous ; we will have to speak of such 

 a species in the following pages. 



A frond will, as a rule, increase in 

 thickness by successive division of its 

 ascending filaments, but in the group 

 of P. (Cruoriella) polystrata we find a 

 thick frond consisting of layers of nar- 

 row fronds lying one above the other, 

 creeping continually over each other 

 and forming a thick frond by this mode 

 of growing. Again in another species the 

 frond, after having acquired a certain 

 thickness, splits or tears in a horizontal 

 direction ; the lower part decays little 



by little, the upper part continues the growth of the frond ; its in- 

 ferior cells grow larger, produce rhizines and develop the characters 

 of an ordinary hypothallus. Such a young thallus, after having ac- 

 quired the necessary thickness, will split in its turn ; I have seen 



Fig. 139. Cruoriella armo- 

 rica. Crn. Filaments of the 

 hypothallus forming little 

 fanshaped groups. (90 : 1). 



