PRINCIPAL GROUPS OF PLANTS. 25 



gial wall, and after swimming about for a time comes to rest 

 and germinates, giving rise to a new plant (Fig. 12, C, D). This 

 large zoospore is multinucleate and multiciliate, there being two 

 cilia for each nucleus, and by some botanists is considered to be 

 an aggregation of numerous biciliate zoospores. It is also of 

 interest to note that the zoospores of Vaucheria appear to arise by 

 a grouping of the cytoplasm and the nuclei already existing in the 

 sporangium rather than by repeated divisions of a single nucleus. 



Another method of reproduction in Vaucheria (Fig. 12, F) 

 is that by means of oospores, or spores formed by the union of 

 egg and sperm cells. Two special branches are formed on the 

 thallus as short side shoots. One of these branches, known as 

 the oogonium (Fig. 12, og), is somewhat egg-shaped and sepa- 

 rated from the thallus by means of a cross wall. It contains a 

 great many chromatophores and considerable oil, and has a com- 

 paratively thick wall. The apex is somewhat beaked and con- 

 tains colorless protoplasm. The second branch, which is known 

 as an antheridium (Fig. 12, h), is smaller, somewhat cylindrical 

 and curved towards the oogonium. It is also cut off from the 

 thallus by means of a cross wall. The antheridium contains very 

 little chlorophyll, but a great many sperm cells. These are oval 

 or egg-shaped and have two cilia, one at each end. The sperms 

 escape from the apex of the antheridium and enter an opening 

 at the apex of the oogonium, one of them uniting with the egg 

 cell, which then develops a thick membrane, the resulting oospore 

 being a resting spore. 



Ulva, or Sea Lettuce, is a common form found all over the 

 world, especially in brackish waters. In its usual form it consists 

 of flat, thin, unbranched fronds which are more or less ovate or 

 orbicular in outline and frequently deeply incised, sometimes be- 

 coming linear or even ribbon-shaped (Fig. n). The fronds con- 

 sist of two layers of cells, which are either in close contact with 

 each other or else at maturity separate so as to form a tubular 

 frond. It sometimes occurs in large quantities in the shallow 

 water along our coast, and is conspicuously disagreeable by its 

 resemblance in shape to the swollen intestines of some animal. 



CEdogonium is a filamentous alga occurring usually in simple 

 unbranched filaments and attached by a disk-like cell or hold- 



