228 'ALG.^E 



heterogamous. In other respects it indicates affinity with Ulothrix, and 

 is surrounded by a remarkably thick lamellated gelatinous envelope. It 

 is made by some writers the type of a distinct family, the Cylindro- 



CAPSACEtE. 



Literature, 



Cohn — Ann. Sc, Nat., v., 1856, p. 1S7. 

 Heinricher — Ber. Deutsch, Bot. Gesell., 1883, p. 433. 



Rauwenhofif — Rev. Internat. Sc. Biol., 1883, p. 176 ; and Arch. Neerl. Sc. Exact, 

 et Nat., 1887, p. 91. 



Class XII.— Fucaceae. 



This family — adopting the limits first proposed by Thuret — consists 

 of a small number of genera of large olive-brown seaweeds distinguished 

 by their mode of sexual reproduction, and by the entire absence, 

 throughout the class, of zoospores, or indeed of any kind of non-sexual 

 spore. 



The thallus or ' frond ' is often several feet in length, cyhndrical or 

 flattened, or, in Himanthalia (Lyng.), cup-shaped, of a cartilaginous 

 texture, and is attached to the sea bottom by a branched rhizoid or 

 attachment-disc. This organ is altogether superficial, and has no function 

 in the absorption of food-material, hke the root of higher plants. It 

 is formed entirely of filaments originating from the stipe or stem. In 

 some species detached branches have the power of maintaining their 

 existence, and even multiplying for an indefinite period, floating on the 

 surface. Nearly all the species are perennial. Although the thallus 

 does not display the same amount of external differentiation into ' stipe ^ 

 and ' frond ' as some of the Laminariace^, the differentiation of internal 

 tissues is quite as great. In the centre of the thallus is a medullary 

 system composed of elongated cells, and surrounded by a cortical system 

 of shorter nearly isodiametrical cells : there is only a very rudimentary 

 development of epiderm. The thallus increases in thickness by the 

 radial division of the outermost rows of cells or hyphce of which the 

 cortex is composed. Growth in length is entirely apical, taking place, 

 according to the most recent observations, by the segmentation of a 

 single well-marked four-sided apical cell, which may be seated, as in 

 Fucus furcatus (Ag.), at the base of a depression at the apex of the 

 frond. Grabendorfer states (Bot. Zeit., 1885, pp. 609 et seq.) that in 

 Durvillaea (Bory) there is no apical growing point. The thallus of the 

 Fucaceae always branches dichotomously and monopodially, the branches 

 lying, when not disturbed, in a single plane. In the more highly 



