Some Aspects of the Alga;. 43 
to the later beginning of the vegetative period. In the Mediterranean 
the summer vegetation is very scanty. Another vegetative maximum, 
especially in Plankton, is reached in the autumn. This double curve 
is attributed, on good grounds, to seasonal light variations rather 
than to those of temperature. The light is supposed to be too 
weak in winter and too strong in summer for active assimilation and 
therefore growth. The heat factor cannot, however, be wholly 
excluded. The striking fact is brought out that the typical green 
freshwater algae aestivate and hibernate in the form of resting cells 
—hypnozygotes or hypnakinetes—while the typical red and brown 
marine forms pass their resting periods in the vegetative condition, 
usually dying down more or less completely—except in the case of 
the “evergreen” Fucaceaeand Laminariaceae—and shootingoutagain 
at the beginning of the next vegetative period. This phenomenon 
is correlated with the more equable temperature of the sea, and it 
Is pointed out that the freshwater Florideae, which inhabit mountain 
streams, are also exposed to less variable temperature conditions 
than the inhabitants of ponds and ditches. 
The chapter on the Phenomena of Irritability is divided into 
two parts, the first dealing with Directive, the second with Formative 
Stimuli. Under the heading of Phototaxy and Phototropism, a 
neat arrangement is figured for shewing that at a certain light 
intensity—the “optimum”—motile cells sensitive to light move 
neither towards nor away from the region of greater intensity, but 
perform a slow upward movement which is probably geotactic. As 
might be expected, the phenomena of response to directive stimuli 
are fundamentally the same as those met with in the higher plants, 
though they are often much less markedly developed. 
Under “Formative Stimuli” a great deal of interesting infor¬ 
mation is brought together, particularly in relation to “Wound- 
Stimuli,” though but little synthesis is possible. Most of the section 
on the effect of conditions on the formation of reproductive cells is 
taken from Klebs’ w^ell-known work. Here also generalisation is 
scarcely attempted, though Klebs’ distinction between “antecedent 
conditions” and “releasing stimuli” is maintained. The difference 
in the reaction of various algae to the same substances is certainly 
often very puzzling, though, we think, an attempt to find a clue 
through at least a considerable part of the labyrinth might have met 
with success. The Palmella forms of various algae are considered 
as relatively “ passive states ” induced by conditions under which 
normal growth or reproduction are no longer possible. 
The next two chapters are very short, and are devoted to 
