SECT. I 



CRYPTOGAMS 



327 



The thallus of Vauckeria, the only oogamous genus of the Siphoneae, also con- 

 sists of a single cell attached to the substratum by means of colourless rhizoids ; 

 but its aerial portion, unlike that of Botrydium, is branched and filamentous. 



The swarm-spores of Vauckeria are developed in special sporangia, cut off from 

 the swollen extremities of lateral branches by means of transverse walls (Fig. 248, 

 A-E). The whole contents of such a, sporangium become converted into a single 

 green swarm-spore. The wall of the sporangium then ruptures at the apex, and 

 the swarm-spore rotating on its longitudinal axis forces its way through the open- 

 ing. The swarm-spore [F) is so large as to be visible to the naked eye, and contains 

 numerous nuclei embedded in an investing layer of colourless protoplasm. It is 

 entirely surrounded with a fringe of cilia, which protrude in pairs, one pair opposite 

 each nucleus (G). Morphologically the swarm-spores of Vauckeria correspond to 

 the collective individual spores of Botrydium. The sexual reproduction of Vauckeria 

 is not effected like that of the other Siphoneae, by the conjugation of motile gametes, 

 from which, however, as the earlier form of reproduction, it may be considered to 

 have been derived. The oogonia and 

 antheridia first appear as small protu- 

 berances, which grow out into short 

 lateral branches and become separated 

 by means of septa from the rest of the 

 thallus (Fig. 249, o, a). At first, 

 according to Oltmanns, the rudi- 

 ments of an oogonium contain numer- 

 ous nuclei, of which all but one, the 

 nucleus of the future egg-cell, retreat 

 again into the main filament before 

 the formation of the separative septum. 

 In its mature condition the oogonium 

 has on one side a beak -like projection 

 containing only colourless protoplasm, 

 while the rest of the oogonium is filled with numerous chromatophores and oil 

 globules. The apical portion of the projection becomes mucilaginous, and is finally 

 ruptured by the extrusion of a colourless drop of protoplasm from the egg -cell 

 which, in the meantime, has been formed by .the contraction of the contents of 

 the oogonium. The antheridia, which are also multinuclear, are more or less coiled 

 (a), and open at the tip to set free their slimy contents, which breaks up into a 

 number of swarming spermatozoids. The spermatozoids, which are very small 

 and entirely devoid of chromatophores, consist chiefly of nuclear substance. They 

 collect around the receptive^- spot of the egg -cell, into which one spermatozoid 

 finally penetrates. After the egg -cell has been fertilised by the fusion of its 

 nucleus with that of the spermatozoid, it becomes invested with a wall and con- 

 verted into a resting oospore. 



The marine Siphoneae, on account of the more complicated segmentation of their 

 thallus, afford one of the most interesting types of algal development. The genus 

 Caulerpa, represented by many species inhabiting the warmer water of the ocean, 

 has a thick, creeping main axis or stem. Increasing in length by apical growth, 

 the stem-like portion of the thallus gives off from its under surface profusely 

 branched colourless rhizoids, while, from its upper side, it produces green thalloid 

 segments which vary in shape in the different species. In Caulerpa prolif era (Fig. 

 250) these outgrowths are leaf-like, are frequently proliferous, and have only a 

 limited growth. In other species they are pinnately lobed or branched. The whole 





fefc^Vteaa. 



Fig. 249. — Vauclieria sessilis. Portion of a filament 

 with an oogonium, o ; antheridium, a ; cli, chro- 

 matophores ; n, cell nuclei ; ol, oil globules. 

 (X240.) 



