THALLOPHYTA. 661 



swelled out with starch, and assume a stellate form (starch-stars). They serve as 

 organs of vegetative propagation. 



The other large genus of Characeae is Nitella. It is especially distinguished by 

 the fact of its stems and leaves being destitute of cortex. There are 67 species of 

 Ghara and 77 of Nitella. 



Characeous fruits (Oyrogonites) are met with in large numbers in the lower 

 chalk and in tertiary formations. Only rarely are fragments of the stems, &c., 

 recognized. 



Alliance XI. — PhseophycesB. 



^is 



Families: Ectocarpacece, Sphacilariacece, Cutleriacece, Laminar iacece, Fucacece. 



Includes the whole series of the Brown Sea-weeds, essentially characterized by 

 the fact that their chlorophyll-corpuscles include, in addition to chlorophyll, a brown 

 pigment, Phycophsein, which masks the green colour of the chlorophyll. The forms 

 included under this alliance are all multicellular, and range from simple threads of 

 cells to large complex forms showing a differentiation into a root-like attachment- 

 organ, stipe, and expanded leaf-like frond. In several of these larger forms the 

 internal structure almost rivals that of Flowering Plants in complexity. Within 

 the limits of the group we find sexual reproduction, in some eases by the fusion of 

 equivalent motile gametes (c/. p. 50), in others of well-marked egg-cells or sper- 

 matozoids. Fertilization and the complete life-history has been studied in relatively 

 few cases. 



Ectocarpacece. — These are mostly filamentous and often branched. Attached to 

 the branches are the sporangia and gametangia. From the former motile zoospores 

 are liberated. From the latter similar bodies — the gametes — are liberated. These 

 possess two cilia, attached laterally to the gametes. The process of fertilization 

 has been followed in Ectocarpus siliculosus. Certain of the gametes come to rest 

 first, and these are approached by a number of other gametes, which swarm around 

 them. Ultimately one of the swarming gametes fuses with the resting gamete and 

 fertilizes it. This process has been thought to indicate the existence of a certain 

 sexual differentiation amongst the gametes, the gamete which comes to rest first 

 being the egg-cell. However, there is no demonstrable structural difference between 

 them. 



SphacelariaceCB. — The filaments consist of many layers of cells. Reproductive 

 organs agreeing in the main with the last-named family. 



Cutleriacece. — Mostly branched, ribbon-like forms. The gametangia are arranged 

 in tufts, and the gametes differ in size, but both possess two cilia. 



Laminar iacece. — Perhaps the most interesting family of the alliance. Many 

 forms are known to liberate motile reproductive cells from various portions of their 

 surface, but the fate of these bodies has not been ascertained. They are large marine 

 forms, some of them attaining gigantic dimensions. Laminaria digitata, which 

 grows in quantities near low- water mark on our coasts (where it forms a regular 

 " Laminaria-zone "), has a tuft of powerful roots holding it to the rocks, a long 



