654 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



Each antheridium then liberates a spermatozoid. Fertilization has not been observed, 

 but there can be no doubt that a spermatozoid passes down the neck of the oogonium 

 and fuses with the oosphere. Subsequently the neck of the oogonium breaks off, 

 the oospore puts on a cell-wall, and the neighbouring cells branch so as to form a 

 closely investing cortex of cells round the oospore. The contents of these cortical 

 cells turn red-brown; and after a resting stage the cortex comes off, the oospore 

 having in the meanwhile divided to form a disc of cells, each of which gives rise to 

 a zoospore, which escapes and germinates to form a new plant. 



The structure of the oogonium, and the formation of a cortex round the oospore, 

 as well as the formation of " carpospores " by the latter, recall the simpler Red 

 Sea- weeds, a group which some authorities consider to be derived from some form 

 like Coleochoßte. 



The indirect formation of new plants by the products of division of the zygote, 

 a phenomenon we have already met with in Pandorina, Hydrodictyon, Ulothrix, 

 (Edogonium, and other forms, is probably the means by which the sporophyte 

 generation of Mosses and Liverworts arose. At first the zygote gave rise at once 

 to spores, but later on a certain amount of sterile tissue was produced in addition, 

 and this formed the body of the sporophyte. 



Alliance IX. — Conjugatae. 



Families: Desmidioideoe, Spirogyracece, Zygnemacece, Mougeotiacece. 



This is a very sharply characterized alliance of Green Algse. It is indeed 

 diflBcult to determine its affinities. The forms belonging to it are especially charac- 

 terized by never forming zoospores, and by possessing aplanogametes, i.e. gametes 

 which, instead of escaping from the mother-cell and swarming freely, never leave 

 the cavities of the cells in which they are produced. When conjugation is about to 

 occur the two cells (gametangia), the contents of which will form gametes, approach 

 one another, and their walls come into contact, either directly, or by the putting out 

 from one or both cells of a short cellulose tube. The area of wall at the place of 

 contact breaks down, and the whole or part of the contents of each cell then fuses 

 with the corresponding protoplasm of the other to form a zygote. 



The chromatophores of the Conjugatae, though very various in the different 

 families, are all very different from the types met with among the other Green 

 Algse. 



I. Desmidioidem. Cell -contents and outline symmetrically arranged on each 

 side of a given median plane which is often coincident with a more or less deep 

 constriction. Often unicellular. 



II. Zygnemoidece. Cells cylindrical, without median constriction, always form- 

 ing threads. 



1. Spirogyracece. Chromatophores one or more, parietal, spiral 



2. Zygnemacece. Chromatojjhores two, axile, roundish. 



3. Mougeotiacece. Chromatophore single, axile, plate-like. 



