272 
THALLOPHYTES. 
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 Acetabularia. 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 ^-^0 to line. 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 SaprolegnieaB ^ 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. 
^ Nägeli in Zeitschrift für wiss. Bot. 1844, Bd. I, p. 134. 
3 [Woronin, Ann. des Sc. Nat. 4® ser., vol. XVI. p. 200. De Bary and Strasburger have 
detected (Bot. Zeit. 1877) the conjugation of zoogonidia.] 
* Nägeli, Die neueren Algensysteme, p. 177. [The renaarkable fossil plant from Canada of 
Devonian age, Prototaxiies Logani, was probably an enormous Siphonaceous Alga ; see W. T. 
Thiselton Dyer, Journ. of Bot. 1871, p. 252 ; Carruthers, Monthly Micros. Journ. 1872, p. 160.] 
5 Pringsheim in Jahrb. für wiss. Bot. vol, I. p. 285 [Ann. d. Sc. Nat. 1859, tom. XI. p. 349] ; 
vol. II. p. 205 ; vol. IX. p. 191.— De Bary, ditto, vol. IL p. 169.— Hildebrand, ditto, vol. VI. p. 249.— 
Leitgeb, ditto, vol. VII. p. 257.— Cornu in Ann. de Sei. Nat., 5th ser., 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.] 
