FRESH-WATEll A L G Jii OF TUE UNITED STATES. 177 



Remarks. — The Vaucheriacea are araongst our most common fresh-water alira;. 

 They occur generally in the form of vast numbers of individuals interwoven into 

 broad mats, which have often both a felty look and feel. When growth is going 

 on rapidly, these mats are of a beautiful vivid green ; but when the process of 

 sexual reproduction has checked the life of the individual they become dingy and 

 dirty looking. The thallus is composed of a single cell and is almost always 

 branched. The branches never have, at least in any of our species, a definite 

 arrangement, save only in that they always arise from the side and not from the 

 point of the thallus. In the European species, V. tuberosa, however, the branches 

 are said to arise both from the point and sides of the frond. 



The frond cell is generally nearly uniform in diameter and has a thick outer 

 wall, which is composed of cellulose, as is proven by the action upon it of iodine 

 and sulphuric acid and of the iodo-chloride of zinc solution. Within the cell are 

 chlorophyllons protoplasm, starch granules, watery fluid, and a few scattered 

 raphides or inorganic crystals. There is never any nucleus. The protoplasm is 

 often very granular, and is mostly collected in a thick green layer upon the inner 

 surface of the cell wall, leaving the centre of the cell free for the more watery 

 contents. 



Growth, except in the very young fronds, consists exclusively in an increase in 

 length, and takes place only at the ends of the thallus or in the portions near it. 

 The branches are almost always simple, but are said in some species to give origin 

 to secondary branchlets, and even, at times, to tertiary ones. They grow in the 

 same manner as the main thallus, /. e. by additions to their ends. 



AVhen the thallus of a Vaucheria is ruptured by external injury, or, at times, 

 when it is dying from some hidden cause, a number of bright green globes of 

 various sizes are formed out of the endochrome. These appear to have the power 

 of independent existence for some time, but whether or not they ever actually 

 grow into new thalli I am unable to state. 



M. Walz asserts ,.th at he has observed in certain species the formation of a quiet 

 spore without the intervention of sexual organs, and that the process is as follows. 

 The end of a long or short twig swells up, and the chlorophyl and protoplasm from 

 the neighboring parts accumulate in the enlarged portion. A partition wall then 

 forms at the base of the latter, which is thus changed into a closed chamber, a 

 sporangium. The- green contents then slowly gather themselves together into a 

 denser and denser ball, becoming more and more separated, in so doing, from the 

 wall of the sporangium, and finally secreting around themselves a distinct mem- 

 brane. After the formation of a spore in this way, the sporangium opens at the 

 apex and allows it to escape. The spore, after remaining quiet for some time in 

 the water, at last germinates into a new frond, in a similar manner to an ordinary 

 zoospore. In my earlier studies of fresh-water algoe, I noticed something very 

 similar to this in one of our species, but convinced myself that the little body was 

 nothing but a zoospore, whose normal development had been perverted by unto- 

 ward influences, and therefore paid no more attention to the matter. It is proba- 

 ble that the life-history of the bodies observed by M. Walz is capable of the same 

 explanation. 



23 August, 1872 



