ommand are taken into consideration. His pupil, De
Candolle, who afterward became so eminent a worker in the same field, when
preparing his "Flora of France," in 1805, proposed the name of _Vaucheria_
for the genus, in commemoration of the meritorious work of its first
investigator. On March 12, 1826, Unger made the first recorded observation
of the formation and liberation of the terminal or non-sexual spores of
this plant. Hassall, the able English botanist, made it the subject of
extended study while preparing his fine work entitled "A History of the
British Fresh Water Algae," published in 1845. He has given us a very
graphic description of the phenomenon first observed by Unger. In 1856
Pringsheim described the true sexual propagation by oospores, with such
minuteness and accuracy that our knowledge of the plant can scarcely be
said to have essentially increased since that time.

[Illustration: GROWTH OF THE ALGA, VAUCHERIA, UNDER THE MICROSCOPE.]

_Vaucheria_ has two or three rather doubtful marine species assigned to it
by Harvey, but the fresh water forms are by far the more numerous, and it
is to some of these I would call your attention for a few moments this
evening. The plant grows in densely interwoven tufts, these being of a
vivid green color, while the plant is in the actively vegetative condition,
changing to a duller tint as it advances to maturity. Its habitat (with the
exceptions above noted) is in freshwater--usually in ditches or slowly
running streams. I have found it at pretty much all seasons of the year, in
the stretch of boggy ground in the Presidio, bordering the road to Fort
Point. The filaments attain a length of several inches when fully
developed, and are of an average diameter of 1/250 (0.004) inch. They
branch but sparingly, or not at all, and are characterized by consisting of
a single long tube or cell, not divided by septa, as in the case of the
great majority of the filamentous algae. These tubular filaments are
composed of a nearly transparent cellulose wall, including an inner layer
thickly studded with bright green granules of chlorophyl. This inner layer
is ordinarily not noticeable, but it retracts from the outer envelope when
subjected to the action of certain reagents, or when immersed in a fluid
differing in density from water, and it then becomes distinctly visible, as
may be seen in the engraving (Fig. 1). The plant grows rapidly and is
endowed with much vitality, for it res