SWARM-SPORES OF CLADOPHORA. 293 



time, and especially if dilute nutrient solution is added, can 

 resume their growth and division. - - 



The process of conjugation we have just studied is character- 

 istic of the entire section of Algae collected together as Conjugates. 

 To this, besides the species of Spirogyra, so widely diffused in 

 fresh water, belong also the almost equally widely-spread species 

 of Zygnema, recognisable by two stellate chromatophores in each 

 cell ; and the elegantly-formed Desmidese. Into close connection 

 with these latter we may even bring the Diatomaceae, in which 

 typical conjugation is likewise present. 



Reproduction of Cladophora Swarm-spores. The genus Cla- 

 dophora, belonging to the Chlorophycese (the structure of which 

 is already known to us) provides us with a right favourable object 

 for the study of swarm-spores or zoospores ; it is only to be 

 regretted that they are not always inclined to their production. 

 It is comparatively easy to obtain swarm spores of the marine 

 forms, which we keep in a large vessel with sea-water ; and amongst 

 fresh-water forms, Cladophora glomerata, if taken from rapidly- 

 flowing water, and laid about evening in a shallow vessel with a 

 layer of water about J inch deep, will be usually found to have 

 swarm-spores next day. The formation of these commences at 

 the apices of the branches, and proceeds towards their base; 

 hence all stages of development are easily found close together. 

 We examine these in acropetal succession, and commence with 

 a still unchanged cell. The structure of this is already known to 

 us (p. 247). So much as is visible without reagents we soon 

 recognise again : the polygonal, closely-crowded chromatophores, 

 containing small, pale, starch granules, in part also having larger 

 pyrenoids ; the plasmic plates which traverse the cavity of the 

 cell, and in part also contain chromatophores. If we pass now 

 gradually from such a cell to such as are being transformed into 

 sporangia, first of all a change of the colour of the contents 

 strikes us ; later on the chromatophores begin to arrange them- 

 selves into a net, so that the entire contents of the cell (which 

 have increased in quantity with proportional diminution of the 

 central vacuole, the contents of which become slimy, and may 

 even be obliterated) appear divided into approximately equal 

 polygonal sections. At one place, usually near the anterior end 

 of the cell, and in terminal cells occupying the anterior end, 

 the cell-membrane swells, and, as the result of its increase in 

 volume, bulges into a papilla-like projection. The chromatophores 



