382 BOTANY OF THE LIVING PLANT 



is as in E. siliculosus (iv.-vi.). A third example is seen in Cutleria, in 

 which the male and female gametangia, as well as the gametes which 

 they produce, differ still more markedly in size (Fig. 323). These steps 

 in sexual differentiation make the condition seen in Fucus intelligible. 



In the Fucaceae there are no means of vegetative propagation, 

 the increase in numbers being wholly by the sexual process. The 

 sexual organs are not exposed, as in the simpler Algae, but borne 



Fig. 320. 



A, Pleurocladia lacustris. Uni- 

 locular sporangium with its contents 

 divided up into zoospores, a = eye- 

 spot. c/tr=chromatophore. (After 

 Klebahn.j B = Chorda filum, zoo- 

 spores. (After Reinke.) (From 

 Oltmanns' Algae.) 



Fig. 321. 



Ectocarpus siliculosus. i, female gamete sur- 

 rounded by a number of male gametes. 2-5, stages 

 in the fusion of gametes. 6, zygote after 24 

 hours. 7-9, fusion of the nuclei as seen in fixed 

 and stained material. (1-5 after Berthold ; 6-9 

 after Oltmanns.) (From Strasburger.) 



in conceptacles, which are cavities hollowed out of the thallus, and 

 clustered at the ends of its branches, thickened to accommodate 

 them (Fig. 319). Similar conceptacles bearing tufts of hairs but no 

 sexual organs are scattered more or less sparsely over the vegetative 

 thallus. In Fucus serratus the male and female organs are borne 

 on distinct plants, but in many species they may appear in the same 

 conceptacles. A median section through a male conceptacle shows 

 how the flask-shaped cavity opens to the outside by a narrow pore, 

 and is at maturity filled by richly branched hairs, which arise from 

 the tissues bounding the conceptacle, and bear the numerous minute 



