590 



SCIENCE 



[N. S. Vol. XXV. No. 641 



microscopic slide. A solution of any desired 

 concentration of radium bromide of known 

 activity is made in a suitable solvent, and 

 applied to the surface of the slide near the 

 center. When the solvent evaporates a film 

 of the salt is left on the slide. The film is 

 protected by a coating of a specially prepared 

 substance. Living cells may now be mounted 

 as on an ordinary slide, and their response, if 

 any, to the stimulus of the rays observed. The 

 coating has the advantage, not only of being 

 sufficiently transparent to light, but easily 

 transparent to the y8 and y rays, and in less 

 degree to the a rays also. 



Slides of various styles and modifications 

 have been prepared on the above principle by 

 Mr. Hugo Lieber, of H. Lieber & Co., New 

 York City, and their efficacy is now being 

 tested in the laboratories of the New York 

 Botanical Garden. Thus far only plant cells 

 have been studied in this way, but the device 

 could doubtless be used in studying the 

 cleavage of eggs, and other activities of animal 

 cells. 



To Discharge Electrified Paraffin Bihlons. — 

 Every user of the microtome has experienced 

 the annoyance arising from the electrification 

 of the paraffin ribbons. The trouble may be 

 easily avoided by any device that will conduct 

 away the charge of electricity as rapidly as it 

 accumulates. If the air were a perfect con- 

 ductor, the trouble would not arise, but its 

 conductivity is greatly increased through ion- 

 ization. This ionization may be conveniently 

 accomplished by supporting, near the place 

 where the microtome-knife cuts the sections, a 

 celluloid rod, covered on one end with Lieber's 

 radium-coating. These rods have been used 

 with great satisfaction by the writer to avoid 

 the difficulty mentioned. 



0. Stuaht Gager 



New York Botanical Garden, 

 New Yobk Citt 



SPECIAL ARTICLES 

 ELEMENTARY SPECIES AND HYBRIDS OF BDRSa' 



The rearing of over 20,000 pedigreed speci- 

 mens of Bursa Bursa-pa-storis (L.) Britten, 



'Extract from paper read before the joint meet- 

 ing of Section F and Section G, A. A. A. S., New 

 York City, December — , 1906. 



has demonstrated the presence of at least four 

 elementary species, all of which breed true 

 when self-fertilized or crossed within the 

 limits of the same elementary form. From 

 over thirty hybrid families the fact is derived 

 that these several elementary forms hybridize 

 in strictly Mendelian fashion, each form 

 which went into the cross coming out again 

 in the perfectly pure extracted dominant or 

 recessive form of the parents. 



The existence of elementary forms in nature 

 within the recognized limits of the species, 

 differing from one another as do the ele- 

 mentary species of Bursa, in the possession of 

 definite characters that behave as hereditary 

 units, presents a condition that is not unique, 

 but one which has an important bearing upon 

 some of the questions that have been recently 

 discussed Several atypic plants have ap- 

 peared in the cultures, which have bred true 

 to their atypic characters, when the assump- 

 tion that they were due to chance crosses 

 would have required that they split into the 

 atypic and typic forms in the ratio 3 :1. 

 These occurred in families of which the pol- 

 lination was not guarded and their status as 

 mutants is in consequence not considered suf- 

 ficiently secure to be presented in detail at 

 present as proofs of mutation. The fact that 

 throughout these cultures the differentiating 

 characters behaved as units in the Mendelian 

 sense appears to me indubitable evidence that 

 the several elementary species have arisen 

 through mutation and hybridization. 



On the basis of allelomorphic differences 

 between different species these elementary 

 species of Bursa represent the closest possible 

 relationship between forms belonging to dis- 

 tinct types, since they are seen in most cases 

 to differ from each other by a single dis- 

 tinguishing unit or by two units at most. 

 The fact that Bursa Bursa-pastoris is every- 

 where recognized as a variable species, and the 

 presence of several distinct forms in each of 

 the localities from which material was derived 

 for these studies, show that these elementary 

 species generally grow in actual contact with 

 each other. There is neither geographical 

 isolation nor complete physiological isolation, 

 yet these nearly related elementary forms 



