252 FUCACEJE. PART TI. 



spores, is contained in thickened club-shaped cells spring- 

 ing vertically between the surfaces of the frond. 



A transverse section of the stem of many of the 

 larger sea weeds presents zones, formed period by 

 period, corresponding with the development of the 

 laminae, roots, and branches. The stem of the Les- 

 sonia bears a strong analogy to that of dicotyledons 

 in having rings of growth, though there is a great 

 difference. As increase in Lessonia takes place by the 

 constant division of a flat leaf, the basilar portion of 

 which becomes the petiole and ultimately swells into 

 a branch, the stems have always a more or less ellip- 

 tical form, and their section exhibits an elliptical core. 

 This form of the core is not however peculiar, but exists 

 in other Algse. It is probable that the Lessonise, although 

 attaining so large a size, are really of rapid growth. 2 



The Ecklonia is essentially a southern genus, though 

 one species ascends to Spain and the Canaries. The 

 frond is pinnatifid, the segments arising from the evo- 

 lution of marginal teeth. The stem of the Ecklonia 

 buccinalis, which is three or four inches thick and 

 strongly inflated above, exhibits rings of growth with 

 an orbicular central pith. 



The group of the Fucacese exhibits the highest struc- 

 ture of all the olive-green Algse, and forms a large portion 

 of the sea weeds on our coasts, but they abound more in 

 individuals than in the number of genera and species. A 

 few have cylindrical stems and branches swelling out at 

 intervals into large oblong inflated air-vessels, which 

 gives them buoyancy in the water. The rest have a 

 flat, ribbon-like stem, and for the most part dichotomous 

 branches with a decided midrib, but no air-vessels, 

 because they chiefly grow at half-tide level, and are ex- 

 posed twice every twenty-four hours. The most common 

 of our fuci, the Fucus vesiculosus, or bladder- wrack, has 



2 Berkeley's ' Cryptogamic Botany.' 



