272 Trow. — Observations on the Biology and 
Most botanists engaged in actual teaching are probably 
familiar with certain difficulties which attend the practical 
study of the genus. In my own experience Pythium de 
Baryanum was commonly met with on dying cress-seedlings 
a few years ago. It is said that the spores of this species 
are universally present in the soil of gardens. Be that as it 
may, it has not put in an appearance at Cardiff during the 
last four years, notwithstanding repeated (annual) efforts 
made to secure it. Pythium de Baryanum is apt to be dis- 
placed by allied forms, and these are, as a rule, much less 
suitable for teaching purposes. In fact, many of them, owing 
to the delicacy of their mycelium and the irregularity with 
which the reproductive organs make their appearance, are 
almost, if not quite, useless for the purposes of demonstration 
to beginners. Even with so good a type as P. de Baryanum 
young students find it most difficult to make passable fresh 
preparations from infected cress-seedlings. It is not generally 
recognized, however, that the genus possesses a number of 
species which are pure saprophytes, and that of the remainder 
many can be readily cultivated as saprophytes. The parasitic 
species offer for most purposes no advantage over the sapro- 
phytic ones, for there is little to interest us in the attack of 
the parasite on its host in this case. Indeed in many species 
of Pythium the parasite apparently attacks its living host 
in just the same way that the saprophyte attacks the dead 
organic matter. The saprophytic species are, however, much 
easier to work with than the parasitic ; for (1) dead organic 
matter is more easily procurable than living organic matter 
in the form of seedlings, and (2) suitable dead organic 
matter can easily be sterilized, while it is very difficult, if not 
impossible, to sterilize such objects as living seedlings, and 
(3) dead organic matter can be kept in the sterile con- 
dition for long periods with ease. So great are the advantages 
that teachers would do well to discard the haphazard methods 
now in vogue of securing cultures of parasitic species (fre- 
quently mixed), and provide themselves with a pure culture 
of a suitable saprophytic species, from which at any time 
