ALG^. 



217 



blue colouring-matter, are again extracted with strong alcohol, a green solution is 

 obtained, which 'may be split up, as Millardet and Kraus have shown, into chlorophyll 

 and phycoxanthine, if a large quantity of benzine is shaken up with it ; this takes up the 

 green chlorophyll and forms when again at rest an upper layer, while the lower alcoholic 

 layer retains the yellow phycoxanthine^. 



The Hydrodictyeae are a small group of Algae to which belong without doubt the 

 genera Hydrodictyon and Pediastrum, and, probably also according to Pringsheim some 

 others whose cycle of development is not yet known. Their cells contain pure chloro- 

 phyll, and are distinguished by forming a large number of swarm-spores, which, when they 

 come to rest, unite into a single family ; this is tabular in Pediastrum ; in Hydrodictyon 

 it forms a wide-meshed sac-like net. They also produce in addition smaller swarm- 

 spores which go through a long period of rest, and, in their further development, give rise 

 to an alternation of generations. From the researches of A. Braun^ and Pringsheim'', 

 the following process of development occurs in the case oi Hydrodictyon utriculatufn, which. 

 lives in stagnant or slowly flowing fresh water. In the mature state the thallome of this 

 plant is a sac-like net several inches long ; the individual cells, united only at their ends 

 and forming four- or six-cornered meshes, are cylindrical and some lines long ; all 

 the cells of a net are sister-cells, formed simultaneously from one mother-cell. The 

 mature cells have a firm compact w^all, clothed with a thick layer of chlorophyll-green 

 protoplasm, and enclosing cell-sap. In reproduction, the green protoplasmic sac in some 

 cells of the net splits up into large naked daughter-cells, whose number reaches from 

 7000 to 20,000; but in other cells into smaller ones numbering from 30,000 to 100,000. 

 Only the first or larger ones form new nets at once ; they move first of all with a trembling 

 motion within the mother-cell for about half an hour, and then form a daughter- 

 net, which becomes free by absorption of the mother-cell-wall, and, under favourable 

 conditions, attains its full size in three or four weeks, the separate cells elongating 400 

 or 500 fold. The smaller swarm-spores, on the contrary, leave the mother-cell and 

 disperse, remaining in motion often for three hours. They are oval, and are fur- 

 nished at the hyaline end with two long cilia ; when finally at rest, they are spherical, 

 and surround themselves with a firm cell- wall. In this state they may remain dried up 

 for months, if protected from light. After remaining several months at rest, these spores 

 begin slowly to grow, and a vacuole is formed in the green protoplasm. At first from 

 T2() to T^o "ini. in size, they attain a diameter of -in mm. Their contents split up 

 by successive divisions into two or four portions, each of which forms a large swarm- 

 spore. After a few minutes they come to rest, each large swarm-spore constituting 

 a peculiar polyhedral cell, the angles of which grow out into long horn-like prolonga- 

 tions. These polyhedra make a considerable growth ; their protoplasm becomes parietal 

 and encloses a large sap-cavity ; it finally splits up again into swarm-spores which move 

 about with a creeping motion for 20 or 40 minutes within the inner layer of the wall of 

 the polyhedron which protrudes from it as a hernioid sac; they then come to rest, 

 and form a hollow net. These nets formed from the polyhedra consist of only from 

 200 to 300 cells, but otherwise behave in the manner described above. In some 

 polyhedra smaller and more numerous swarm-spores are formed, but these also unite 

 into a net. 



The Volvocineee* are, during the whole of their vegetative period, continually in 



1 Millardet and Kraus, Comptes Rendus, LXVI, p. 505. [See also Sorby, Proc. Roy. See. XXI, 



.P- 457-] 



2 A. Braun, Verjiingung, p. 146.— Ray Soc, Bot. and Phys. Mem. 1853, pp. I37. 190- 



3 Pringsheim, Men. der konigl. Akad. der Wiss. zu Berlin. Dec. 13, i860. [Quart. Joum. 

 Micr. Sc. 1862, pp. 54, 104.] 



* Cohn, Ueber Bau und Fortpflanzung von Volvox glohator in Berichte iiber die Verhand. der 

 schlesischen Ges. fur vliterland. Cultur, 1856 (also in the Comptes Rendus, vol. XLIII, Dec. i, 1856, and 

 Ann. des Sci. Nat. 1857, p. 323).— Cohn, Ueber Chlamydococcus und Chlamydomonas (Protococcus), 



