646 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



certain branches of the latter swarmers of two sizes ai'e produced. Analogy would 

 lead us to suppose that at least the smaller of these are gametes, but their behaviour 

 has not been observed. Some species of Codium (e.g. C. tomentosum, with a dichoto- 

 mously branched furry thallus) occur on our own coasts. The remaining Codiacece 

 are largely tropical, but very widely distributed. 



Valoniacece. — We may include in this family an assemblage of genera whose 

 thallus consists of a branching tube, usually forming transverse walls, but with no 

 interweaving of the branches such as we get in Codiacese. 



The simplest type is found in Valonia, a form which at first consists of a single 

 club-shaped cell, which produces a whorl of branches at its upper end. Each of 

 these may again produce a whorl of branches of the second order. 



A group of very beautiful genera form leaf -like structures, the branching of the 

 thallus taking place in one plane. Struvea consists originally of a single cell, which 

 grows apically and becomes divided by transverse walls into a series of segments. 

 Each of these segments bears a pair of branches coming off right and left, and each 

 branch behaves like the main axis. The secondary and tertiary branches thus pro- 

 duced come into contact, fixing themselves one to another by means of curious little 

 rosette-like organs called tenacula, and the whole thus forms a net-like structure 

 with larger or smaller meshes between the branches. The definite usually oval form 

 of the " leaf ", of which the main axis forms the midrib, and the primary branches 

 the principal veins, is due to the latter, after they have attained a certain length, 

 ceasing to produce branches on the side towards the base of the thallus, and at the 

 same time bending forwards and inwards to join the primary branch next in front. 



Struvea delicatula sometimes lives in the tissue of a Sponge belonging to the 

 genus Halichondria. There is a reciprocal effect on the form of the two organisms, 

 the Alga only taking on the characteristic Struvea-iorm when part of its thallus 

 grows out clear of the body of the Sponge. In consequence of this it was for some 

 time not suspected that this sponge-inhabiting Alga had any connection with 

 Struvea. Neither organism seems to suffer from the association, and there is some 

 evidence for regarding it as a case of true symbiosis. 



Anadyomene is another very beautiful form resembling Struvea in the construc- 

 tion of its thallus, but with no meshes between the cells. It consists of two kinds 

 of cells, the more elongated form the "ribs" of the thallus, the smaller and more 

 rounded make up the intermediate tissue. 



The genus Boodlea, in which the branching takes place in more than one plane, 

 forms a transitional form connecting these genera with Gladophora, which is usually 

 regarded as belonging to the Confervoidese. 



Verticillatce. — In this group of the Siphonese the thallus consists of a long 

 cylindrical undivided stalk, fixed below by rhizoids, and bearing above acropetal 

 whorls of simple or branched appendages of limited growth. In some of these 

 appendages gametes maybe produced; zoospores are apparently absent. It includes 

 two sub-families, the Acetabulariese and Dasycladese. 



Acetahulariece. — Fertile and sterile appendages distinct. Acetahularia mediter- 



