August 17, 191 1] 



NATURE 



209 



A TEXT-BOOK ON HELIOTROPISM. 



Light and the Behaviour of Organisms. By Prof. 

 S. O. Mast. Pp. xi + 410. (New York: J. Wiley 

 and Sons; London : Chapman and Hall, Ltd., ign.) 



THE question of the influence of light upon 

 organisms must have appealed to the inquiring 

 mind ever since a moth was seen to fly into a candle 

 or a plant was observed to bend towards the sun. The 

 closing of many flowers at night coinciding with the 

 opening of others would have also been arresting 

 movements to many unknown observers. But how 

 subtle the influence of light may be has only compara- 

 tively recently been appreciated. The bending of plants 

 towards the sun may readily be explicable on the 

 ground of the importance of light in the preparation 

 of the plant-food, but the hydroid zoophytes were dis- 

 covered to possess the same property of bending 

 towards the source of light, although they do not find 

 thereby any known assistance to their maintenance. 

 Larvae of many diverse marine animals are also 

 strongly attracted by light, and these, again, make 

 no use of it so far as is known. Many animals, 

 indeed, prefer rays of a particular region. As Lord 

 Avebury showed years ago, the common Daphnia, if 

 covered by a spectrum, aggregates under the green 

 and yellow rays ; ants, on the other hand, aggregate 

 chiefly under the red and green rays, showing a 

 special avoidance of violet and ultra-violet rays. Such 

 varied and definite susceptibility seems quite unintel- 

 ligible unless it is connected in some way with well- 

 being, and no one has as yet shown any such correla- 

 tion. The problems, then, of the meaning of the 

 attraction or repulsion which light exerts are evidently 

 very diverse, and the work of Mr. Mast is devoted to 

 their analysis and consideration. 



The first section contains an interesting historical 

 summary of observations chiefly upon plants. Ray 

 was apparently the first to suggest an explanation of 

 the movements of plants, which he attributed to the 

 flow of sap, being attracted to the problem by the 

 sensitive mimosa then recently introduced from 

 America. Early in the nineteenth century de 

 Candolle reversed the daily periodic sleep-movements 

 of leaves by exposing them to a new light rhythm 

 — an interesting anticipation of F. Darwin's work. 

 Many other points of interest are raised, including a 

 discussion of the use of such terms as "tropism," 

 originally the relation between the bending of a plant 

 and the source of stimulation. 



In part ii. the author sets out an account of his 

 experiments on certain plants (Plumules of Indian 

 corn and leaves of Tropa?olum), and arrives at the 

 conclusion that differences of light-intensity are re- 

 sponsible for the movements (observed by a special 

 "light-grader"), but the method of regulating the 

 movements is still a mystery. The section on the 

 light-responses of unicellular plants and animals in- 

 cludes some original observations, as does that upon 

 coelentcrates and various larval forms of higher animals. 

 The general summary of this section is given on 

 pp. 228-235, ar, d includes a useful analysis of the 

 work of many writers. The third part of the book is 

 NO. 2 1 8l, VOL. 8;1 



concerned with general considerations — the adaptive 

 nature of light-responses, the phenomenon of aggre- 

 gation in different intensities, and so on. This section 

 and the last (which deals with reactions of animals 

 to different wave-lengths) are very diffuse, and might 

 well have been considerably shortened, as in many 

 cases the discussions are not capable of leading to 

 any definite solution ; and if the excuse be offered that 

 "behaviour" in its broadest sense was the subject of 

 treatment, then surely the problem of colour-adapta- 

 tion might have been mentioned. The points that 

 emerge from this work are that there are many 

 striking movements towards or away from the source 

 of light which are quite unintelligible unless animal 

 metabolism is favourably effected thereby, and we 

 have no reason to suppose it is; and, secondly, that 

 the way in which these orientations are effected is 

 equally obscure. The book is written by a pupil of 

 Jennings, who has done so much to analyse the be- 

 haviour of the lower organisms. It contains a very 

 useful bibliography, and should prove helpful to that 

 increasing number of experimental biologists and 

 psychologists who are interested in the behaviour of 

 organisms. F- ^ • *-*. 



PLEASANT PATHS OF NATURE. 

 The Airy Way. By George A. B. Dewar. Pp. vii + 

 253. (London : Chatto and Windus, 1910.) Price 

 6s. net. 



IN a delightfully breezy volume Mr. Dewar plays 

 lightly on the word "airy " in its varying appli- 

 cation, straying, by the way, into the rapid-running 

 waters— the' fishes' airy way. Following two most 

 interesting chapters on flight and one on the watery 

 way, the contents of the rest of the volume may be 

 sufficiently indicated by the chapter headings dealing 

 with the airy moor and links, the fritillaries' airy 

 way, the airy lane and common, rooks in the airy way, 

 and "my airiest downs "—a year's observations on 

 the chalk. Field naturalists will be grateful to the 

 author for his gift of conveying impressions in a few 

 words— impressions which many may have felt, but 

 could not perhaps express so aptly. 



A few quotations from this pleasant book will serve 

 to illustrate the pertinence of his remarks and the 

 accuracy of his observations. Of the swift, a bird 

 for which he has a great admiration, he says that it 

 is flight, and is fitted with two scythes to cut and 

 sweep through the air. Of the start of birds from 

 the ground he writes : 



"Hide behind a hedge or wall, and, when every 

 pigeon beak is down, tap your stick on the ground. 

 As you tap — whilst sound 'is still coming from the 

 stone — every bird is in the air. You would as soon 

 try to reckon the time between the hammer striking 

 the cap and the discharge of the cartridge as reckon 

 the time between the stick striking the stone and the 

 discharge of the pigeon party." 



And of the salmon : 



"No man swims if a salmon swims. The keeping 

 afloat, and the slow movements of a man in compara- 

 tivelv still water, should not be given the name we 

 give to the swift, sure, glorious action of the salmon 

 in the fury of storming streams. Does a man who 



