178 PRESH-WATBR ALG^ OF THE UNITED STATES. 



Although I have very frequently cultivated Vaucherias, I have never been so 

 fortunate as to see them form their zoospores, nor indeed to see a zoospore in its 

 motile state. The life-history of these bodies has, however, been fully and repeat- 

 edly worked out by other observers. It is described by such as occurring in the 

 following manner. One end of a branch first enlarges into a bulbous, often conical, 

 point, into which the neighboring endochrome crowds itself. This point is next 

 divided off by a partition wall from the remainder of the thallus and constitutes the 

 zoos'porangium, the contents of which rapidly condense into one or two masses, 

 generally oval in shape, each of which eventually forms a zoospore. When the 

 latter are matured, the apex of the zoosporangium opens, and the little bodies 

 within slowly and gradually emerge, without any apparent cause for their motion. 

 Sometimes, according to Cohn, instead of this steady outward passage, there are 

 repeated forward and backward movements of the zoospores within the case. The 

 zoospore after its perfection is generally oval, and very large. Within it there are 

 one or inore vacuoles, and surrounding it is a layer of colorless protoplasm. It is 

 remarkable for having its whole surface densely covered with short cilia. Its period 

 of motile life appears to be very brief; according to Walz, that of the zoospore of 

 V. sericea, Lyngb., lasts only from one-half to one and a half minute, after which 

 time the cilia are lost and a cellulose wall secreted around the mass. Germina- 

 tion takes place by the growth of the cylindrical thread out from each end of the 

 zoospore. 



True sexual reproduction takes place in this family by means of antJieridia and 

 oogonia, male and female organs. All known specits are mostly if not absolutely 

 monsecious, both organs being contained in the one individual and always placed 

 in proximity. All of the species in which the development and structure of the 

 sexual organs have been studied, agree in the essential points. 



The first appearance of the antheridium is as a little pouch projecting out from 

 the side of the thallus. This increases in size and soon assumes the peculiar shape 

 of the species. At the same time there is a diminution, according to M. Walz, of 

 the chlorophyl in the antheridium, so that, when the partition wall forms and shuts 

 off the cavity of the latter from that of the thallus, there are only a very few scat- 

 tered green granules remaining. The antheridium at the time of separation con- 

 tains, therefore, only transparent protoplasm, which soon becomes granular, and 

 shortly afterwards exhibits the moving spermatoz'oids, which appear to be formed 

 out of the thick layer of protoplasm that lines the inner surface of the cell wall. 

 The point of the antheridium opens so soon as the spermatozoids are perfected, 

 and allows them to escape. 



The formation of the oogonia takes place very similarly to that of the antheridia. 

 There is the same little protrusion from the side of the thallus in the commence- 

 ment of the process, the same after-growth and increase of this pouch, and the same 

 formation of a separating wall between it and the main body of the frond. A very 

 marked difference, however, is to be found in the contents of the two, the oogonium 

 from the very commencement being crowded with chlorophyl and oil globules. 

 When the oosporangium is completed, the end of it opens, and, at the same time, 

 the contents gather themselves into a dense protoplasmic ball, which lies in the 



