Chapter VII 



EUPHYCOPHYTA 



PHAEOPHYCEAE {continued) 



DICTYOSIPHONALES3 DESMARESTIALES, 

 LAMINARIALES5 FUCALES 



DiCTYOSIPHONALES 



The members of this order are fundamentally parenchymatous in 

 construction, the thallus either being of a simple or branched radial 

 habit or else a flattened leafy structure with intercalary growth. 

 There is usually a medulla of large cells with a narrow cortical zone 

 of smaller cells on which are borne the assimilatory hairs and 

 sporangia. In some cases {Asperococcus, Colpomenid) there is sub- 

 sequent degeneration of the large internal cells giving a tubular or 

 saccate thallus. The members show heteromorphic alternation, the 

 macroscopic plant being diploid, usually arising as a lateral out- 

 growth from a plethysmothallus. 



Punctariaceae: Petalonia (Phyllitis, Ilea). Fig. 90 



The unbranched fronds are expanded, membranous, leaf-like 

 structures with an internal medulla composed of large, colourless 

 cells interspersed with hyphae, and an outer layer of small, super- 

 ficial, assimilatory cells. Unilocular sporangia are not known nor are 

 there any paraphyses. The plurilocular sporangia, which are ar- 

 ranged at right angles to the surface, arise from the superficial cells 

 and produce zooids that germinate to give a creeping basal thallus 

 from which a new plant arises. It is therefore suggested that the 

 plants are wholly diploid and that the haploid generation has been 

 lost. Yendo (1919), however, has reported that these zooids can 

 develop after a resting period into minute protonemal threads 

 bearing antheridia and oogonia which presumably produced 

 gametes, although no sign of fertilization was observed. If these 

 observations are correct this genus must be regarded as anomalous, 



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