244 ~ ALG^ 



and is septated by transverse divisions; it is constantly dying off at the 

 apex, the growing point lying at its base immediately above the rhizoids. 

 More often the upper part of the thallus is differentiated into branched 

 annual 'leaves' of cartilaginous texture, usually flat, but sometimes 

 tubular, and often ribbed. Lessonia (Bory) grows erect to a great 

 height, and resembles a branching tree with pendent leaves two or three 

 feet long (fig. 214). In Thalassiophyllum (Post), Agarum (Grev.), and 

 other genera, the frond is beautifully perforated ; these perforations are 

 formed from hollow conical papilte by w^hich the frond is first covered ; 

 the tissue diminishes at the apex of the cones, then bursts, and the 

 opening enlarges as the frond grows. In Macrocystis (Ag.) the stalk-like 

 base of each branch of the frond is swollen out into a large pear-shaped 

 air-bladder. In Nereocystis the air-bladder is barrel-shaped, six or 

 seven feet in length, and crowned with a tuft of fronds. Sieve-hyphae or 

 trumpet-hyph^ with imperfect sieve-plates occur in all the genera ; and 

 Oliver has discovered in the comparatively weak stems of Nereocystis 

 and Macrocystis a structure almost identical with that which occurs in 

 the weak climbing stems of many Flow^ering Plants, true sieve-tubes with 

 perfectly formed sieve-plates both in the septa and in the longitudinal 

 cell-walls, provided with a true callus-formation (fig. 217). 



Zoosporanges of one kind only — the unilocular — are at present known 

 in the Laminariace^ ; these are distributed uniformly over the surface 

 of the thallus or are collected into sori^ and are interspersed w^th simple 

 unsegmented club-shaped sterile hairs or paraphyses. Of the mode in 

 w^hich the zoospores act as propagative organs very little is known. 

 Areschoug has observed the germination of the zoospores of Chorda 

 tomentosa (Lyng.) after the coherence of two of them by their beaks ; but 

 he does not regard this as a true process of conjugation. Gardiner be 

 lieves that he has detected the conjugation of zoospores in Alaria (Grev.). 



Along w^ith the Fucacese, the Laminariaceas are one of the most im- 

 portant commercial sources of iodine. The species of our own shores 

 are employed in the manufacture of kelp. Alaria esculenta (L.) is used 

 by the inhabitants of Scandinavia and Iceland as an article of food, as 

 also are Laminaria digitata (Lmx.) and other species under the name of 

 ' tangle.' The stems of the last-named species are employed for surgical 

 purposes ; those of Ecklonia (Hornem.) and others of the larger genera 

 are used as siphons and for making fishing-nets. 



Literature. 



Reinke — Pringsheim's Jahrb. wiss. Bot., 1876, p. 317, 



Areschoug— Observ. Phycol. , iii. , 1875, iv. , 1883, v., [884 ; and Acta Soc. Sc. Upsa- 



liensis, 1875, 1883, and 1884. 

 Will — (Macrocystis) Bot. Zeit. , 1884, pp. Soi et seq. 



