Sept, 19, 1878] 



JNATURE 



543 



be of the greatest value. The readers of Nature will 

 appreciate anything that helps the scientific worker. Now, 

 at last, we are going to the root of things in biology, and 

 only the embr>'ologist knows fully what a boon an egg- 

 hatcher, convenient and easy to work, would be. The 

 eggs of the hen will be wanted in their various stages of 

 incubation, as long as there are workers in these depart- 

 ments, but many other sorts of oviparous animals have 

 to be worked out in all their stages besides the common 

 fowl. Snakes, lizards, tortoises, crocodiles, all these are 

 rivals of the bird in their embryology, and of many 

 kinds the eggs could be procured and their embryos 

 developed if the worker had some such apparatus as Mr. 

 Christy is bringing out. We want, not merely the general 

 embryology of these ovipara, such as is so excellently 

 illustrated and described in Messrs. Foster and Balfour's 

 work, but the special development of any important organ 

 ought to be traced in all its stages through not one, but 

 many types of the vertebrata : through all the principal 

 kinds indeed. 



Some of us are trying to do this in the skeletal struc- 

 tures ; the nervous system, still more important, wants an 

 army of workers, then there arc the respiratory, digestive, 

 excretory, and generative organs, all these want a complete 

 history in all their stages, not in one type merely, but in 

 scores of types. We therefore wish well to all energetic 

 and enterprising men who put it into our power to work 

 on a wider scale ; such means and appliances as can be 

 brought out by men too restless for close and patient 

 study, may be of infinite service to the close and patient 

 student, who is too dreamy and abstracted to invent for 

 himself. W. K. P. 



NOTES ON SOME NATAL PLANTS 



GROWING plentifully among the grass on the coast 

 hills of Natal is a small blue flower belonging to 

 the Rubiaceas. In this plant, generally speaking, there 

 are two forms only, in one of which the five stamens are 

 exserted considerably beyond the tube of the rotate 

 corolla, and the stigma is included in the tube ; in this 

 form the tube is almost devoid of hairs. In the other 

 common form the position of these essential organs is 

 reversed, the stigma protruding to about the same extent 

 that the stamens do in the first mentioned, and the 

 stamens being included ; here, however, the upper part 

 of the corolla tube is thickly covered with downy hairs — 

 of course this is an ordinary dimorphic plant. But I find 

 lately a third form of the same species (only, however, 

 rarely) in which both stamens and stigma are exserted 

 and are of the same length, so that here self-fertilisation 

 must take place, as the stamens and stigma touch at the 

 time the former dehisce. I do not think this can be 

 termed a cleistogamic form, as, although rather smaller 

 and lighter in colour than the others, the difference is 

 only trifling. The hairs which cover the corolla-tube in 

 the form with included stamens serve to keep the pollen 

 collected near the upper part of the tube, as, if it fell to the 

 base it would not be so easily transferred by the proboscis 

 of an insect as when lightly held by the hairs through 

 which the insect must make way. As these hairs would 

 be for this purpose useless when the stamens are exserted 

 they do not occur in the other form. 



I notice the same arrangement of hairs in another 

 dimorphic plant belonging, I think, to the same order, 

 which grows on the marshy flats near the sea. I have 

 found on the coast lands here four other plants, in which 

 cross-fertilisation is secured by dimorphism, one of them 

 being a monocotyledonous plant. 



There is a species of Polygonum which climbs in the 

 bush which well illustrates another plan ensuring cross- 

 fertilisation ; while the flower is young and the perianth 

 still closed, enveloping the immature stamens, the three 

 branching stigmas protrude from between the segments 



in a fit state to receive the pollen. If (as is usual) the 

 ripe stigmas were only exposed when the flower opens, 

 although the evils of self-fertilisation would of course be 

 avoided by the plant being proterogynous, still, as it is 

 wind-fertilised, the perianth and stamens would be in the 

 way of any stray pollen-grains reaching the stigmas ; 

 while as it is, nothing interposes between pollen and 

 stigmas. 



Lately I have found a curious aberration of form in 

 Tecoma capense growing here. It is very common in the 

 bush, forming great beds of bright colour, and normally 

 has a scarlet trumpet-shaped corolla, with one rudimentary 

 and four perfect stamens. I found, however, three or 

 four plants growing within a short distance of each other, 

 in which there were eight perfect stamens ; they seemed, 

 however, to have been formed at the expense of the 

 corolla, for there was only one segment coloured at all, 

 the remainder being colourless and small. The ovary 

 seemed in several cases to have been fertilised. The 

 ordinary form of this plant, although individually so 

 brightly coloured, growing in large numbers and secreting 

 much nectar, is seldom or never visited by Lepidoptera. 

 It is, however, frequented by honeysuckers and small 

 bees in numbers. All through the day you can hear the 

 shrill chirp of the small bright honeysucker among the 

 blossoms. The immediate reason why butterflies and 

 moths do not visit it I cannot give ; but the stamens and 

 stigma (which are beneath the large upper segment of the 

 corolla) are long, and so high above the opening of the 

 corolla-tube that those insects, in visiting the flower for 

 its nectar, would not be at all certain to touch either, and 

 so in comparison to the honeysucker and small bees 

 would be of little benefit to the plant ; for when the former 

 of these visits the flower the feathers of his head are just 

 of the height to brush off the pollen, and the latter in 

 collecting the pollen is equally certain to distribute it, 

 as the bifid stigma is about the same length or only 

 slightly longer than the stamens. Can the nectar have 

 been modified to suit the taste of the useful honeysucker 

 without reference to the useless butterfly ? 

 Natal, June 27 M. S. Evans 



PHYSICS IN PHOTOGRAPHY^ 

 III. 



THESE last experiments were remarkable in another 

 point of view, as they opened out the question as to 

 whether the salts of silver might not prove sensitive to 

 rays to which they had been supposed hitherto to be 

 insensitive. Silver iodide, for instance, when exposed to 

 the spectrum in a solution of potassium sulphite proved 

 sensitive as far as " a " of the spectrum instead of 

 stopping short at the point indicated in Fig. 2 (p. 529) ; 

 and silver bromide in the molecular grouping which 

 absorbed the red proved sensitive to a wave-length of 

 somewhere near 11,000, whereas in its normal state 9,600 

 was its limit. 



Similarly silver chloride proved sensitive to an extent 

 which presumably may be increased till it is equal to that 

 of the bromide. In both these instances we have a proof 

 that the compound was sensitive to these abnormal 

 rays, and that the image formed by those rays was 

 destroyed as soon as formed by their oxidising action 

 giving an undevelopable form of salt. It may be re- 

 marked that by exposing films in reducing solutions such 

 as ferrous sulphate, and pyrogallic acid rendered very 

 slightly alkaline, that an image can be developed as fast 

 as it is formed. 



The natural outcome of the experiments on the oxida- 

 tion of the photographic image just narrated is that it 

 should lead to the solution of the problem of photo- 

 graphy in natural colours, such as that of Becquerel, 

 Niepce de St. Victor, and others. In the fourth edition 



» Continued from p. 531. 



