78 ME. J. B. CAERITTHERS ON THE CYSTOCARPS OF 



small, linear to broad, and generally dichotomous. The young 

 thallus in the dry state is of a reddish-brown colour, and adheres 

 pretty firmly to the paper. The cystocarps are produced in 



varying numbers over the whole of the upper parts of the thallus 

 and scattered quite irregularly over the segments. 



This form of thallus is not in accordance with the typical form 

 of Rhodymenia palmata ; and this fact goes to support the opinion 

 that this specimen is not to be considered as Rhodymenia palmata ; 

 but since there are so many varieties of this very common alga, 

 it is possible that it is a specimen of a new variety, and no doubt 

 the determination was made with that notion. Of greater 



importance is the study of the anatomical structure of the 

 thallus. 



The first thing which strikes the attention in a transverse section 

 through the thallus is the existence of two distinct parts — a 

 thick centre or medullary cell-layer, and on both sides a compa- 

 ratively thin cortical layer. The centre part consists of a layer 

 of two or three large round or oval cells, which are arranged in no 

 regular order, but lie parallel to the surface of the thallus. The 

 spaces between the large cells (which, owing to their round form, 

 are not adjacent at all parts) are filled everywhere by small 

 round or long cells of varying sizes according to the interstices, 

 but joined in continuous branches. 



On both sides of the centre part is a layer of smaller cells, 

 generally one fifth as thick as the medullary layer, the cells of 

 which are much smaller than the interstitial cells of the centre 



part, so that a clear line of demarcation can be drawn between 



the two portions of the thallus. This outer portion, in its turn, 

 can be divided into an outer and an inner portion — the outer 

 portion consisting of a layer of very small cells, the inner portiou 

 formed of a layer of larger cells. The outermost row of cells are 

 very close together, forming a compact cortical layer. 



These points of the anatomical structure of this alga show that it 

 cannot be considered as Rhodymenia palmata, Grev. That alga (a 

 specimen of which from the Bay of Bathus— Kabenhorst, Alg. Eur, 

 No. 1745—1 have examined) has only a single layer of large 

 cells in the centre or medullary part, and no interstitial cells 

 between them. That the position of this specimen is not in the 

 genus BJiodymenia is thus to be seen, but to determine exactly 

 its generic position it is necessary to study the fructification. 



The cystocarps are small flat lenticular swellings, and on the 



