SECT. I 



CRYPTOGAMS 



331 



The outer cell layers, as a rule, function as an assimilatory tissue, 

 the inner cells as storage reservoirs. In some species the axial cells 

 of the thallus are arranged in definite strands with sieve-tube like 

 elements and true sieve-tubes. 



According to the manner of their sexual and asexual reproduction, 

 the Phaeophyceae fall naturally into three orders. 



Order 1. Phaeosporeae 



In this order are included the Laminarias, as well as the majority of the other 

 Phaeophyceae. Asexual multiplication is effected by means of swarm-spores, which 

 are produced in large numbers in simple, so-called unilocular sporangia ; they have 

 a red eye-spot, a ehromatophore, and two laterally inserted cilia (Fig. 254). 



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Fig. 254. — Cladosteplms verticillatus. A , Closed sporangium Fig. 255. — Cladosteplius verticillatus, 



( X 280) ; B, swarm-spores escaping from a sporangium 

 (X280) ; C, a single swarm-spore (x circa 2000), with red 

 eye-spot ap, and yellow ehromatophore cTir. (After 

 Pbingsheim.) 



with gametangium partly dis- 

 charged. (After Fringsheim, x 

 500.) 



Many genera exhibit also a sexual mode of reproduction resulting from the 

 conjugation of isomorphous planogametes, which, except that they fuse in pairs in the 

 formation of zygotes, otherwise resemble the asexual 

 swarm-spores (Kg. 256). Unlike the swarm-spores, 

 however, they are produced in many-chambered, 

 plurilocular gametangia, in each cell of which 

 seldom more than one gamete is formed (Fig. 255). 



The members of this order afford an illustration 

 of a transition from isogamy to oogamy. In the 

 small family of Cutleriaceae, to which belongs 

 Zanardinia collaris, whose thallus is disc-shaped 

 and attached at the centre, and the Cutlerias with 

 a furcately-divided thallus, the conjugating gametes 

 are of unequal size. The female macrogametes are 

 much larger than the male microgametes, and have their origin, one in each cell 

 in larger and fewer-celled gametangia. After swarming, the female gamete loses 



Fig. 256. — Ectocarpussilioulosus. a, 

 Gametes ; b, c, fusion of two 

 (After Berthold.) 



