38o 



BOTANY OF THE LIVING PLANT 



Cutleiieae, to the latter the Dictyotaceae and Fucaceae. Examples 

 selected from these illustrate probable steps in sexual differentiation. 

 The propagative cell of the Brown Seaweeds is very constant 

 in its form. It is actively motile in the water into which it is set 

 free. It is a pear-shaped, nucleated protoplast, with two cilia 

 attached laterally so that one is directed forwards the other backwards. 

 They are inserted close to a red eye-spot, which as a rule is closely 

 related to a yellow chromatophore (Figs. 320, B\ 324, 3). The constancy 

 of this type shows the probable phyletic unity of the Brown Algae. 



B 



FIG. 318. 



A, B, successive stages in origin of a conceptacle of Fucus. Incidentally they 

 show the active division of the cells of the superficial layer (I, I) and its production 

 of internal tissues. ( x 530.) F. O. B. 



In the simpler types propagation may be either vegetative, carried 

 out by zoospores produced in unilocular sporangia (Fig. 320, A) \ or 

 sexual, carried out by gametes, produced in plurilocular sporangia 

 (Fig. 323). In form and motility the zoospores and gametes are alike ; 

 but the former are larger and germinate directly into new plants with- 

 out conjugation, while the latter are smaller, and conjugate before 

 germination. It seems probable, therefore, that the gamete is really 

 a zoospore specialised for conjugation. 



In Ectocarpus siliculosus all the gametes appear alike, but they are 

 distinguished as male and female by their behaviour. These are 

 produced from distinct sporangia, which may be borne on the same or 

 on different plants. Those gametes which are functionally female and 

 receptive soon lose their motility, becoming fixed to a substratum 



