306 



KNOWLEDGE. 



August, 1913. 



light is utilised, as atmospheric unsteadiness continually moves 

 its image off the slit. With the prism before the object-glass 

 there is little loss of light, no guiding is required ; the spectra 

 are given any desired width by allowing them to trail on the 

 plate. 



APPEAL FOR MORE VOLUNTEERS FOR OBSER- 

 VATION OF VARIABLE STARS.— The June number of 

 Publications of the Astronomical Society of the Pacific 

 has an article on this subject by Mr. E. Gray. He points 

 out what a suitable field this is for amateurs with a small 

 equipment ; a good binocular suffices to observe the brighter 

 variables, while a very large field of work is open to the 

 possessor of a three-inch telescope : it need not be equatorially 

 mounted, though if not it is necessary to commit to memory 

 the field surrounding each star studied, so as to be able to 

 identify it without loss of time. For the brighter variables the 

 observer can make sketches of the fields for himself ; for the 

 fainter ones he should procure Father Hagen's Atlas. The 

 article quotes the case of an observer who had only a three- 

 inch on a tripod stand who was able to observe twenty-six 

 variables on one evening and to finish work at 8.50 p.m. 

 There is the charm of uncertainty about this work, for while 

 some of the stars are regular, others are subject to large 

 irregularities, and there is the possibility that assiduous work 

 may find the law of these fluctuations. X Monocerotis is 

 quoted as a typical star for which more observers are needed 

 to follow it through all its stages (it does not go below the 

 tenth magnitude). Beginners are more likely to persevere if 

 they take up work that they know is of real utility than if 

 they merely do aimless star-gazing. A further attraction is 

 added to this field of work by the rich red colour of many 

 of the stars. 



THE PLANET ALBERT. — The same publication contains 

 an account of Dr. Haynes's work on this planet (better known 

 as MT). He is taking the three undoubted observations of 

 1911, October 3rd, 4th, and 11th, and combining them one 

 by one with each of the other eight doubtful places, so as to 

 get a series of orbits one of which is presumably right. Search 

 will be made in all possible positions when a favourable 

 opposition recurs. The planet was in opposition last spring, 

 but only of magnitude 19 or 20. Search was, however, made 

 with the Crossley Reflector at Lick, and three faint planets 

 found, but it appeared that none of them could be Albert. 

 I am glad to note that Dr. Haynes has received the degree 

 of Doctor of Philosophy in the University of California in 

 recognition of this work. 



THE TROJAN GROUP OF PLANETS.— The same 

 magazine has an article on this group by Dr. S. Einarsson. 

 They nearly conform to the equilateral configuration with the 

 Sun and Jupiter, which Laplace showed to be an exact 

 solution of the three-body problem. The present paper shows 

 that it is both simpler and more accurate to deal with the 

 motion on this, basis from the first than to treat Jupiter's 

 action as a mere perturbation. 



BOTANY. 



By Professor F. Cavers, D.Sc, F.L.S. 



THE "AFTER-RIPENING" OF SEEDS. — In many 

 cases seeds require a long time for germination, usually 

 owing to the exclusion of water or of oxygen by the seed- 

 coat. After excluding such cases, however, there are some 

 plants left in which the seed does not grow, even when the 

 coat has been removed and the embryo put in good germinating 

 conditions, until a certain period has elapsed, during which 

 some change is evidently taking place in the embryo. To 

 such cases the term " after-ripening " is strictly applied, and 

 Miss S. Eckerson has just made a careful study of this 

 phenomenon (Bot. Gaz., Vol. LV). It has been found that 

 seeds of various pines show a delay in germination of as much 

 as two years, while those of ash sown in spring do not 

 germinate until the following spring. In the latter case the 

 embryo in the mature seed occupies about half the space 



within the endosperm, the rest being occupied by a muci- 

 laginous substance ; during the year that the seed lies in the 

 ground the embryo grows in length and fills the seed-coat ; 

 hence a period of growth is necessary before germination. 



The authoress has studied the delayed germination or 

 " after-ripening "of the hawthorn (several speciesof Crataegus). 

 She finds that food is stored in the embryo in the form of 

 fatty oil ; lecithin is also present, but there is no starch or 

 sugar. The reaction of the cotyledons is acid, but the 

 hypocotyl is slightly alkaline. During the after-ripening period 

 there is a series of metabolic changes in the embryo, beginning 

 with increased acidity, and correlated with this there is an 

 increased water-absorbing power, and also an increase in the 

 activity of the ferments catalase and peroxidase. Near the 

 end of this period there is a sudden increase in the acidity and 

 in the water content ; here oxidase first appears. All of these 

 increase until the hypocotyl is three to five centimetres long ; 

 then fats decrease and sugar appears, while hydrocyanic acid 

 is present in the cotyledons. The after-ripening period can 

 be greatly shortened by treating the embryo with dilute acids ; 

 the water- holding power, the acidity, and the amount of 

 peroxidase increase much more rapidly, and oxidase appears 

 much earlier, than in untreated embryos. Apparently there 

 is a correlation between the acidity of the hypocotyl of 

 Crataegus, its water-absorbing power, production of ferments, 

 and germinating power. Whether the acidity is causal or 

 merely correlative is not known, though there is some evidence 

 that it is causal — for instance, it has been found to lead to the 

 liberation of ferments, and to increase the water-absorbing 

 power of colloids. 



BACTERIAL NODULES ON LEAVES— All students 

 of plant life are doubtless familiar with the remarkable 

 nodules found on the roots of Leguminosae and a few other 

 plants (Aluus, Myrica, Podocarpus, and so on) and con- 

 taining bacteria which fix free atmospheric nitrogen and thus 

 supply the " host " plant with nitrogenous food. Faber 

 (Jahrb. f. wiss. Bot., 1912) describes an interesting and 

 unexpected symbiosis of a similar kind which he has dis- 

 covered in two tropical genera (Pavetta and Psychotria) 

 belonging to the Rubiaceae; but here the nodules are developed 

 on the leaves instead of the roots. The bacteria have the 

 same power of nitrogen fixation as those in the roots of 

 Leguminosae. They are already present in the unopened 

 bud, lying in the cavity formed by the stipules of the leaf, and 

 they enter the leaves themselves by way of the stomata, 

 collecting in the air cavity just below a stoma and multiplying 

 so as to disturb the shape of the surrounding cells of the leaf, 

 which appear to be stimulated to vigorous division, resulting in 

 the formation of a mass of small-celled tissue. In this tissue 

 there are relatively large intercellular spaces, and in these the 

 bacteria grow, though without apparently injuring the cells. 

 After the bacteria have entered and set up the formation of 

 the nodule or "gall," the stoma becomes occluded, shutting 

 them in ; the plant has, so to speak, swallowed the bacteria. 

 The tissue in the swelling contains abundant chlorophyll and 

 starch grains, but when the nodule is fully formed the starch 

 disappears and is replaced by reducing sugar, evidently formed 

 by fermentation of the starch. Finally, towards the close 

 of the leaf's life the bacteria have largely disappeared from 

 the intercellular spaces and the cells again contain abundant 

 starch ; hence the food apparently serves for the nutrition 

 of the bacteria — as one might say, in return for the service 

 rendered by them to the plant in supplying the latter with 

 nitrogenous compounds manufactured from the atmosphere. 

 In a variegated form of Pavetta indica these remarkable leaf 

 nodules are conspicuous as green swellings on the otherwise 

 white leaf. 



The author believes that this represents an ideal case of 

 symbiosis between bacteria and higher, plants, though the 

 latter gets the better exchange, since ultimately the cells of 

 the leaf, whether or not they actually cause the death of the 

 bacteria, at any rate absorb their dead remains ; the walls of 

 the bacterial cells become slimy, and eventually they disappear. 

 The bacteria, which closely resemble the tubercle bacillus, 

 occur in the seed, lying between the embryo and the endosperm, 



