JuLy 29, 1915] 
NATURE 
595 

exhaustible problem of growth, and, lastly, the 
conditions which determine or which prevent the 
entrance of the spermatozoon into the egg. 
In Prof. Loeb’s book, which bears the title set 
at the head of this short article, there is a well- 
known chapter on heliotropism, that is to say on 
the tendency of plants to turn towards the light, 
and on other kindred phenomena manifested both 
in plants and animals; for the polypes of a hydroid 
colony such as Eudendrium, or the tube-dwelling 
worms such as Serpula or Spirographis, also bend 
towards the light, and if they be illuminated by 
a single beam they grow steadily in the direction 
of its rays. It is characteristic of the plant (or 
rather of the green plant) that it represents a 
peculiar type of machmé ‘which is capable of turn- 
ing radiant energy into chemical energy, and so 
ultimately into mechanical work; and in the case 
of the plant, “the permanency of this kind of 
machine is guaranteed by the presence of an 
automatic arrangement, whereby their stems turn 
towards the light.”” So Loeb is inclined to read 
into this phenomenon what we might call a modi- 
fied teleology, such indeed as, in one form or 
another, refuses to be kept out even of our most 
modern biological speculations. 
In the chapter already alluded to, published 
some nine years ago, our knowledge was said to 
be very scanty as to the relative heliotropic effi- 
ciency of the various parts of the spectrum, little 
more being known than that the more refractive 
rays, the green, the blue, and the violet, were 
more effective than the yellow and the red. 
“There exists thus, apparently,” said Prof. Loeb, 
“aq division of labour, the longer light-waves 
accelerating assimilation, and the shorter waves 
accelerating heliotropism ;’—just (we might say) 
as there is obviously a “division of labour” be- 
tween the rays which illuminate and those which 
warm us. 
Two of Prof. Loeb’s recent papers (by Dr. H. 
Wasteneys and himself) (Pr. Nat. Acad. of 
Science, January, 1915, and Science, February 26, 
1915) are devoted to the question of the identity 
of heliotropism in plants and animals, and thereby 
to an inquiry into the particular wave-lengths of 
light to which, in one case or another, they are 
most sensitive. Instead of exposing the organisms 
to the solar spectrum itself, as had been done in 
earlier and simpler experiments, the writers (using 
a carbon are spectrum) allowed the light from par- 
ticular portions of it to pass through narrow slits, 
and then reflected it in a monochromatic beam upon 
the subject of their experiments. They soon 
arrived at the simple but very remarkable result 
that there are two particular regions of the 
spectrum the rays of which are especially effective 
in causing organisms to turn, or to congregate, to- 
wards them; these regions lie (1) in the blue, in 
the neighbourhood of a wave-length of 477 pp, 
and (2) in the yellowish-green, in the region of 
X=534 pu; and these two wave-lengths affect 
different organisms, with no very evident relation 
to the nature of these latter. Thus the blue rays 
(of 477 pp) attract the infusorian Euglena, the 
NO. 2387, VOL. 95] 


hydroid Eudendrium, and the seedlings of oats; 
while the yellowish-green rays (of 534 pm) in turn 
affect the protozoon Chlamydomonas, the little 
water-flea Daphnia, and the larve of barnacles. 
These facts add a quality of precision to many 
older and vaguer observations, for instance, to 
Paul Bert’s discovery (in 1869) that Daphnia 
swims towards the light in all parts of the visible 
spectrum, but most rapidly in the yellow or the 
green. The particular wave-length of 534 ju is 
especially remarkable, because it coincides with a 
determination by Trendelenberg that the visual 
purple of the rabbit’s eye (which is not affected by 
red and very little by yellow light) is bleached 
most rapidly by light the wave-length of which is 
536 pp. It would seem, according to Loeb, that 
among the lower organisms we have to deal with 
two separate photosensitive substances, which 
determine their heliotropic reactions; that these 
are distributed without regard to the systematic 
boundaries, even between plants and animals; and 
that one of the two, occurring even among very 
lowly organisms, has characters similar to, and is 
perhaps identical with, the visual purple of the 
highest type of eye. 
In another paper (Science, November 6, 1914), 
still dealing with the effects of light, Prof. Loeb 
relates some remarkable observations on the action 
of ultra-violet rays upon unfertilised eggs, adding 
by these new experiments a curious detail to the 
many facts regarding artificial fertilisation, by 
chemical or physical means, which we associate 
with his name as their prime discoverer. On the 
ground that ultra-violet rays are known to have 
a sterilising effect, that is to say to be capable 
agents in the destruction of cell-life, and that, 
according to Loeb’s own experiments, the very 
substances which induce “cytolysis” in the living 
cell are also capable (under proper conditions) of 
producing artificial parthenogenesis, Loeb thought 
it likely that these rays would also prove to be 
effective stimuli, under the appropriate conditions, 
of parthenogenetic development. He exposed the 
unfertilised eggs of a sea-urchin for ten minutes 
to the light of a quartz mercury arc lamp, and 
found that the majority of the eggs formed “‘fertili- 
sation membranes”; when kept cool they further 
proceeded to segment, but ere long perished; an 
addition of hypertonic sea-water, however, enabled 
them to develop into larvee, though few advanced 
beyond the gastrula stage. It was remarkable 
that a cover-glass, o°1 mm. thick, was sufficient 
to prevent all action on the part of the light, a 
fact which bears closely on the wave-length of the 
rays which produced the action. 
The discovery here mentioned is certainly in- 
teresting as a specific case of the effect of radiant 
energy on living protoplasm ; but its precise degree 
of interest turns largely on the importance which we 
may attach to the formation of a surface-membrane 
as an essential preliminary to the development of 
the egg. According to Loeb this phenomenon is 
of supreme importance; for the question why an 
unfertilised egg cannot grow, and why a fertilised 
egg can grow and divide, depends, according to 
