144 PHAEOPHYCEAE 



construction. The oblong unilocular sporangia are borne at the 

 base of the assimilatory filaments, but plurilocular sporangia are 

 unknown. When this genus comes to be investigated it will prob- 

 ably be found to have a life history similar to that of the other 

 Mesogloiaceae. 

 CoRYNOPHLOEACEAE : Leathesiu (after G. R. Leathes). Fig. 97. 



The present genus provides an example of degeneration in the 

 cable type of construction. The young plant arises from a small, 



Fig. 97. Leathesia difformis. A, plants on Furcellaria fastigiata. B, transverse 

 section to show thallus construction ( x 24). C, unilocular sporangia (X336). 

 D, plurilocular sporangia (x 336). (A, after Oltmanns; B-D, after Newton.) 



creeping, rhizomatous portion and is composed of a packed mass of 

 radiating, dichotomously branched filaments which are sufficiently 

 closely entwined to make the plant mass solid. From these medul- 

 lary filaments there arises a cortex of densely packed assimilatory 

 filaments. The young plants are subspherical at first, but with 

 increasing age the central medullary filaments commence to disin- 

 tegrate and as a result the mature thallus becomes hollow and 

 irregularly lobed. Plurilocular and unilocular sporangia are known, 

 the zoospores from the ovoid unilocular sporangia germinating to 

 disk-like plantlets 'on which plurilocular gametangia ultimately 

 appear. These plantlets either give rise to other similar plantlets or 



