272 THALLOPHVTES. 



a hyaline prolongation is subsequently formed which branches like a root and penetrates 

 the earth, while the upper part swells up into an ovoid vesicle, in which the proto- 

 plasm forms a parietal layer containing chlorophyll. From this arise, after growth 

 is completed, a number of zoogonidia which are set free by the wall of the mother- 

 cell becoming gelatinous and deliquescing. This is evidently a more simple mode of 

 growth than that of Vaucheria, A higher degree of branching is found in Bryopsis, which 

 is also unicellular. This genus also forms on one side root-like organs of attachment, on 

 the other upright much-branched stems (several centimetres in height) with unlimited 

 growth at the apex ; small branches with limited apical growth are formed on them in 

 two rows or spirally, which clothe the stem like leaves, and after they have become 

 shut off from it by septa, fall off; in them numerous zoogonidia are formed^. The 

 branching of a single large cell is carried still further in the genus Caulerpa, which 

 forms creeping stems growing at the apex with descending branched rhizoids and ascending 

 leaf-like branches 2. The growth of a unicellular thallus takes place in still another 

 manner in Aeetabularia. Here the plant, two or three centimetres high, has the form 

 of a slender Hymenomycetous fungus, the stem of which developes a rhizoid below and 

 bears a pileus above, consisting of a disc of closely crowded rays, which are themselves 

 radial branches of the stem. The stem ends above in the form of a boss ; at the base of 

 the radial branches surrounding the boss stands an umbellate whorl of branched seg- 

 mented hairs. In the rays of the pileus are formed the non-sexual zoogonidia ^. Finally 

 Udotea cyathiformis must be mentioned here. This species forms a stalked leaf-like 

 thallus, the stalk \ inch, and the thallus from \ inch to 2 inches long and broad, its 

 thickness from t^o to ^V I'ne. When cut transversely it seems to consist of a cellular 

 tissue, but in reality the thallus is composed of a great number of branched filaments, 

 which, forming a cortical and medullary layer, are all ramifications of a single cell ^. 



FORMS NOT CONTAINING CHLOROPHYLL. 



2. The SaprolegniesB ^ are colourless parasites usually found attached to animal 

 or vegetable organisms in water, especially dead insects, forming dense radiating tufts. 

 The individual plants are long undivided filaments, penetrating deeply into the substratum 

 by means of root-like branches, and branching more or less in the surrounding water, 

 sometimes in an arborescent manner. The zoogonidia are formed in the branches 

 after the contents have been separated by a septum ; occasionally a number of such 

 septa are formed, and they are then produced in each cell. The zoogonidia are 

 formed simply by the simultaneous division of the contents into a very large number 

 of portions (Fig. 178, A)\ the cell opens at the apex and the gonidia are expelled; 

 these at once swarm about in the water and become dispersed, or at first accumulate 

 in a resting state in front of the opening; each gonidium then becomes immediately 

 invested with a firm cell-wall, but after a short time it abandons it and then begins 



^ Pringsheim, on Bryopsis, in the Monatsber, der Berlin. Akad. May 1871. 



^ Nageli in Zeitschrift fvir wiss. Bot. 1844, Bd. I. p. 134. 



2 [Woronin, Ann. des Sc. Nat. 4° ser., vol. XVI. p. 200. De Bary and Strasburger have 

 detected (Bot. Zeit. 1877) the conjugation of zoogonidia.] 



* Nageli, Die neueren Algensysteme, p. 177, [The remarkable fossil plant from Canada of 

 Devonian age, Prototaxites Logani, v^^as probably an enormous Siphonaceous Alga ; see W. T. 

 Thiselton Dyer, Journ. of Bot. 1871, p. 252 ; Carruthers, Monthly Micros. Joum. 1872, p. 160.] 



^ Pringsheim in Jahrb. fiir wiss. Bot. vol. I. p. 285 [Ann. d. Sc. Nat. 1859, torn. XI. p. 349] ; 

 vol. II. p. 205 ; vol. IX. p. 191. — De Bary, ditto, vol. II. p. 169. — Hildebrand, ditto, vol. VI. p. 249. — 

 Leitgeb, ditto, vol, VII. p. 257.— Cornu in Ann. de Sci. Nat., 5th sen, vol. XV. p. 328. — Schenk, 

 Bot. Zeit. 1859, p. 348. — Pfitzer in Monatsber. d. Berlin. Akad. May 1872. — [De Bary und Woronin, 

 Beit, zur Morphol. u. Physiol, d. Pilze, IV, 1881.] 



