14.
Scientific Memoirs by
regard to the morphological characters of their constituent elements, but in
certain other features. Certain forms are characterised by the frequency with
which the growth on the surface of the nutrient medium presents .a highly adhesive
or ropey consistence, whilst in others it is invariably entirely devoid of any such
peculiarity. During the entire period in which they have been cultivated forms
α , β , ι , λ
h a v e n e v e r p r o d u c e d g r o w t h s s h o w i n g a n y m a r k e d a d h e s i v e n e s s , a n d ,
as a rule, the material is absolutely diffluent, whilst forms ζ, η, ĸ,
ν ξ
a l m o s t
invariably yield growths of a highly' adhesive or ropey character, They are not
invariably so, but this is only to be looked for considering the unequivocal
evidence which we possess that relatively slight variations in the constitution
of the medium are capable of giving rise to modification of property in this
respect, but the invariable absence of the character in the growths of the one
series of forms and the extreme frequency with which it occurs in those of the other
affords fair ground for separating them into two distinct groups differing from
one another in physiological properties. Had all the forms which normally give
adhesive surface growths in neutralised agar-agar media invariably done so, there
would really have been no stronger ground for ascribing the difference between
the two groups to specific peculiarities than there is as the facts actually stand,
for the more evidence we have that the property of yielding adhesive growths
is a modifiable one related to variations in the constitution of the environment,
the stronger does the proof of the existence of profound physiological difference
between the two groups become, as it shows that in one of them no modification
of property arises under the influence of conditions which cause modifications
in the other. In the case of two forms, γ and ε, conspicuous and seemingly
permanent modifications of property in this respect appear to have occurred,
but these have not been in the same direction, for γ, which for long was incap-
able of giving rise to adhesive growths, now constantly produces them, and ε,
which originally possessed the power in very high degree, has for the past
eighteen months absolutely lost it, so that, although we have clear evidence
that in these cases modification has attended continued exposure to like con-
ditions, we have no evidence that the latter has produced any assimilation of
character in the modified organisms.
Certain other characteristic peculiarities of habit constantly present them-
selve in the growths of some of the forms in neutralised agar-agar media. The
surface-growth in form a is constantly distinguished by the excessively smooth,
shining, varnished appearance which it presents, by the relatively slow rate
at which it extends whenever the temperature falls below a certain level, or
the medium is of unusually dense consistence, and by' the highly aggregant
character of its constituent elements, which refuse to diffuse evenly in fluid but in
large numbers adhere to one another so as to form dense clumps. In forms
β a n d
ε ,
on the other hand, the surface-growth is irregular and often conspi-
cuously rough and the elements have little or no tendency towards aggregation.