PH.-EOSPORE.r. 



24T 



of the Laminariacece and of other large marine Algce belonging to other 

 groups display remarkable elasticity or other properties to enable them 

 to resist the traction of the waves. In the larger species the frond is 

 buoyed up by air-bladders. 



Janczewski describes the occurrence in the class of three distinct 

 modes of growth, viz. — (i) The 

 thallus and all its ramifications 

 terminate in a generative apical cell 

 which divides in a direction parallel 

 to its base, and thus gives birth to a 

 series of segments. This occurs in 

 the Sphacelariace^ and in Dictyo- 

 siphon, but is the least common 

 mode. {2) By peripheral growth, 

 i.e. the marginal cells of the thallus 

 are the youngest, and are more or less 

 united into a generative peripheral 

 zone (Myrionema, Grev., Leathesia, 

 Gray, Ralfsia, Berk.). (3) By in- 

 tercalary growth. This is much the 

 most common mode, and there are, 

 again, three modifications of it, viz. 

 — I. The thallus terminates, when 

 young, in one or more hairs, the 

 common growing point of the 

 thallus and of the hairs being situ- 

 ated at their point of junction (Ecto- 

 carpus, Desmarestia, Lmx., Carpo- 

 mitra, Ktz., Cutleria, Sporochnus, 

 Ag.). 2. The thallus is differen- 

 tiated into three 'organs' — frond, 

 stipe, and rhizoids ; the growing point from which the stipe and frond 

 originate is common to these two organs, while the rhizoids increase by 

 apical growth (Laminariaceae). 3. The absolutely undivided thallus is 

 regenerated from the growing point situated at the base of the frond 

 (Scytosiphon, Chorda, Punctaria, Grev., Asperococcus, Lmx.). 



Fig. 214. — Lcssonia J-uscescetis Bon* 

 (greatly reduced). 



Literature. 



Magnus — Festschr. Gesell. naturf. Freunde, Berlin, 1873. 



Areschoug— Bot. Notis. , 1873. 



Gobi-Bot. Zeit., 1877, p. 425. 



Reinke — Ibid., p. 441. 



Thuret & Bornet— Etudes Phycologiques, 187S. 



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