FERTILIZATION AND FRUIT-FORMATION IN CRYPTOGAMS. 



51 



size and form between ooplasts and spermatoplasts. The thallus in all species of 

 Fucus is tough and leatheiy, brown in colour, foliaceous, and dichotomously branched 

 or lobed, and has interspersed here and there air-containing swellings which serve 

 as floats. The apices of the lobes are punctate, and each spot corresponds to an 

 internal cavity which has the form of a globular pit (see fig. 202^). Sections 

 through these cavi- 

 ties show that a 

 large number of 

 segmented filaments 

 known as " para- 

 physes " spring 



from the lining- 

 layer of the cavity. 

 In Fucus vesicu- 

 losus (figs. 202 and 

 203) these filaments 

 remain concealed in 

 the cavity; in some 

 other species of 

 Fucus they pro- 

 trude through the 

 narrow orifice (osti- 

 ole) of the cavity 

 in the form of a 

 pencil of hairs. 

 Amongst the fila- 

 ments other struc- 

 tures are also 

 formed within the 

 cavity. A few of 

 the cells lining the 

 cavity swell into 

 papillae, and each 

 becomes divided by 

 the intercalation of 

 a transverse septum 



into two cells, one of which is spherical, whilst the other assumes the form of a 

 stalk bearing the upper one (see fig. 202 ^). The protoplasm in the spherical cell is 

 dark brown, and breaks up into eight parts, which round themselves off and con- 

 stitute the ooplasts. The thick wall of the spherical cell resolves itself into two 

 layers, of which the inner one incloses the eight rounded protoplasmic bodies like 

 a bladder. This bladder stufied full of ooplasts next detaches itself entirely, and 

 glides upward between the paraphyses until it reaches the orifice of the cavity. 



Fig. 202.— Fucus vesiculosus. 



1 Longitudinal section through one of the cavities in the thallus. 2 a. vesicle surrounded by 

 paraphyses from the bottom of the cavity. 3 a detached vesicle containing eight 

 ooplasts ; the inner lamella swollen up. * Liberation of the ooplasts from a rent vesicle. 

 (After Thuret.) 



