THALLOPHYTA. 



663 



this stalk are attached a series of long ribbon-like leaves, each of which, just at its 

 point of insertion upon the stem, swells into an air-bladder about the size of a 

 pigeon's egg. Thus the stem, which is attached below, is buoyed up, and the long 

 leaves depend into the water. In structure the stem is not unlike that of a Lami- 

 naria; but it possesses in addition to the medulla, with its trumpet hyphse, a zone 

 containing large 

 sieve-tubes, which 

 resemble those con- 

 tained in the soft 

 bast of a Flower- 

 ing Plant (cf. vol. i. 

 fig. 10*, p. 45, and 

 fig. 125 ^ p. 469). 

 Nereocystis, occur- 

 ring on the W. 

 coast of N.America, 

 consists of a long 

 stalk (attaining to 

 a length of nearly 

 100 metres), at- 

 tached at its lower 

 extremity and ex- 

 panding above into 

 a huge retort- 

 shaped air-sac, from 

 the surface of which 

 a number of fronds 

 (6-10 metres in 

 length) arise. Like 

 Macrocystis, its 

 stem contains well- 

 marked sieve-tubes. 

 It is used by the 

 Aleutians as fish- 

 ing-line. Of La- 

 minariacese about 

 90 species have been distinguished (including 30 species of Laminaria). 



Fucacece. — Includes a number of the larger common sea- weeds. They are 

 characterized — like the last family — by a segmentation into a well-marked shoot 

 and organ of attachment. The former is usually flattened and branched, and often 

 bears air-bladders. Reproduction is by means of spermatozoids and non-ciliated 

 egg-cells, which arise in flask-shaped hollows (conceptacles) on definite portions of 

 the shoot or frond. Asexual reproduction by detachment of fragments. 



376.—FIICUS vesiculosus. 



1 Vertical section through a female conceptacle. 2 a single oogonium from the conceptacle 

 surrounded by sterile hairs, s A detached oogonium containing 8 egg-cells ; the inner 

 lamella of the wall is much swollen. * Liberation of the egg-cells. 1 x 50 ; 2, 3, 4 x 160. 

 (After Thuret.) 



