PART IV. — VEGETABLE TAXONOMY. 



of Dicotyledons. The sporangia, which are of 

 the unilocular variety, are borne on the lamina ; 

 and though probably the plants reproduce sex- 

 ually, the fact of conjugation has not yet been 

 observed. 



Fig. 500 represents a plant of Laminaria 

 saccharina, about one-thirtieth natural size. To 

 this group also belong the Ectocarpese, the 

 Sphacelarieae, the Lessonias and the gigantic 

 Microcystis. 



The Fucacese are dark olive-green algse of 

 considerable size, and having a cartilaginous 

 consistency. Most of them adhere firmly to 

 rocks by means of branching aiscs, but a iew 

 float free in the ocean waters. The vegetative 

 body consists either of a dichotomously branch- 

 ing, more or less flattened thallus, as in the 

 genus Fucus, or of fairly differentiated stems 

 and leaves, as in Sargassum. Many of the 

 species possess air-bladders, which render the 

 branches buoyant. 



They differ from the previous group, in not 

 producing swarm-spores, as well as in their 

 more complex mode of sexual reproduction. 

 This takes place by means of antheridia and 

 oogonia. 



We may take the common Bladder-wrack, 

 Fucus vesiculosus, as the type of the group. 

 A portion of the plant is shown in Fig. 501. It 

 grows attached to rocks between high and low 

 tide. The frond is flattened, cartilaginous, two 

 or three feet in length, and repeatedly forking. 

 It has a prominent midrib, and on either side of 

 it, at intervals, air-vesicles occur in pairs. The 

 fruiting organs occur at the ends of certain 

 branches, in cavities called conceptacles, which 

 are arranged close together, and consist of 



fe r 



Fig. 

 ral size. 



500. — Laminaria saccharina, about one-thirtieth natu- 



