82 SEAWEEDS 



thick, pitted walls, which appears to discharge the 

 special function of a supporting tissue. In Macrocystis 

 it is the inner portion of this layer that develops into 

 sieve-tubes, of which the sieve-plates become endued 

 with a callus-like coating. By the periodical addition 

 of such tissues the old stalks of Laminariacew 

 acquire the appearance of the stems of woody 

 Dicotyledons. Occupying the central portion of 

 the stalk there is a dense plexus of branching 

 anastomosing filaments, which probably acts as a 

 conducting tissue. It is gradually reduced in bulk 

 upwards, and enters the midrib of the leaf, when 

 present, as a thin strand. In several Laminariacece 

 there are formed definite mucus-passages, mostly in 

 the form of branching anastomosing tubes, and these 

 are frequently bordered by other cells differentiated 

 from those of the adjoining tissues. 



The Reproductive Organs are zoospores produced in 

 elongate, sac-like, unilocular sporangia of the same 

 shape as those of the Splachnidiacece. They occur, as 

 has been mentioned, in more or less localised sori, 

 mixed with paraphyses. On the one hand, in Chorda, 

 the sporangia occur over the whole surface ; on the 

 other, in Alaria and its allies, they are restricted to 

 special sporophylls, as described above. In Postelsia 

 and Macrocystis the sporangia are borne in the longi- 

 tudinal furrows in the fronds. The paraphyses, unlike 

 those of Splachnidium, are non-septate, and in all 

 cases, except Saccorhiza and Chwda, possess a peculiar 

 hyaline appendage at the tip. Reinke has placed 

 Chorda outside the Laminariacece on account of its 

 cylindrical thallus and the distribution of its spor- 



