88 



CHLOROPHYCEAE 



although one is found in Europe, including Great Britain. The 

 thallus is composed of richly branched threads ramifying in the 

 intercellular spaces of the host. As a result of the presence of the 

 endophyte the chloroplasts of the host cells do not develop and 

 yellow-green patches occur on the leaf, whilst at the same time the 

 adjacent cells may be stimulated to active division resulting in gall 

 formation, but later on the affected cells die. Reproduction takes 

 place by means of oval aplanospores. 



Fig. 60. Fig. 61. 



Fig. 60. Phyllosiphon Arisari. A, leaf of Arisarum vulgar e with whitened patches 

 due to attack of alga. B, portion of thallus ( x 66). (A, after Fritsch; B, after 

 Smith.) 



Fig. 61. Bryopsis. A, plant of B. plumosa ( x o-6). B, portion of same enlarged 

 ( X 7). C, B. corticulans, rhizoid formation from lower branches. D-F, stages in 

 septum formation at base of gametangium. ^ = gelatinized material, ;« = mem- 

 brane, r = ring of thickening initiating septum. G, B. plumosa, female gametangia. 

 c = chloroplast. (A, B, after Taylor; C-G, after Fritsch.) 



*Caulerpaceae: Bryopsis {bryo, moss; opsis, an appearance). 

 Fig. 61. 



Most of the species of this genus are restricted to warmer seas, 

 though at least two, of which B. plumosa is the commoner, occur 

 in colder waters. The principal axis, which is often naked in its 

 lower part, arises from an inconspicuous, filamentous, branched 

 rhizome that creeps along the substrate and is attached to it by 

 means of rhizoids. In one species the bases of the lower branches 



