February 15, 1907] 



SCIENCE 



263 



loides the hymenium was at first covered 

 by a delicate membrane or veil, which it 

 breaks through. The writer's observations 

 on several species confirm those of Dietrich. 

 It is probable that in all members of the 

 group the hymenium is at first covered. 



Notes on the Embryology of BhizopJiora 

 Mangle: Mel. T. Cook, Santiago de las 

 Vegas, Cuba. 



Presents many difficulties in technique, 

 owing to the presence of tannin, etc. Only 

 ene of the four ovules develops. Integu- 

 ments develop early and increase the diffi- 

 culties. Embryo-sac apparently follows the 

 normal course. Embryo has a long fila- 

 mentous suspensor, the synergid end of 

 which degenerates very early, but the op- 

 posite end persists very late. Endosperm 

 very abundant and the outer cells grade 

 almost imperceptibly into the nucellus cells. 

 The cotyledons develop very early at the ex- 

 pense of the endosperm cells and have the 

 appearance of being very much cramped. 

 After the development of the cotyledons is 

 well advanced the root tip begins to grow 

 more rapidly. At the same time the sur- 

 face cells of the cotyledons become very 

 much modified and very protoplasmic, evi- 

 dently for absorption. Cells through the 

 entire embryo show evidence of great ac- 

 tivity, the vascular system becomes more 

 prominent and peculiar ingrowths from the 

 surface of the cotyledons connect with it. 

 Endosperm almost entirely disappears and 

 the embryo is fed by the mother plant. 



The Embryology of Rhytidophyllum Crenu- 

 latum and R. Tomentosum: Mel. T. 

 Cook. 



Of interest because the family is tropical 

 and subtropical and unworked morpholog- 

 ically. Ovule anatropous. Archesporium 

 a single subepidermal cell which usually 

 does not divide but elongates and produces 

 the normal eight-nucleate sac. Synergids 



small and staining deeply. Antipodals 

 small and disintegrating early. The polar 

 nuclei unite near the antipodals. Micro- 

 pyle and pollen tube conspicuous. En- 

 trance of pollen tube obliterates the syner- 

 gids. Endosperm undergoes primary divi- 

 sion before the first division of the proem- 

 bryo, and the two daughter nuclei are sepa- 

 rated by a cross wall which soon disappears. 

 Both nuclei divide rapidly. Micropylar 

 endosperm disintegrates very early. Em- 

 bryo at first filamentous, followed by a 

 division of the apical cell. Suspensor elon- 

 gates and appears to function as a haus- 

 torium, then disintegrates. Differentiation 

 of tissues does not follow the exact order 

 of Capsella bursa-pastoris and other dicot- 

 yledonous plants. 



Radioactivity a Factor in Plant Environ- 

 ment: C. Stuakt Gagee, New York Bo- 

 tanical Garden. 



The hitherto recognized factors of plant 

 environment may be classified as molar (in- 

 cluding living organisms), molecular and 

 undulatory. Radioactivity is an expres- 

 sion of atomic disintegration, and is accom- 

 panied by a relatively enormous release of 

 energy. At least four different factors may 

 be included under the term radioactivity, 

 for the breaking down of the atom of a 

 radioactive substance is accompanied by 

 (1) a stream of negatively charged ions or 

 'corpuscles,' each about one thousandth the 

 size of a hydrogen atom, and moving with 

 about 95 per cent, the velocity of light. 

 Streams of corpuscles constitute the alpha 

 rays; (2) a stream of positively charged 

 ions, of nearly twice the size of a hydrogen 

 atom, and ^moving more slowly than the 

 corpuscles. Streams of positive ions con- 

 stitute the beta rays; (3) an electro-mag- 

 netic pulse, analogous to the X-ray, and 

 caused by the starting or stopping of ions. 

 These rays, given off from radioactive sub- 

 stances, are termed gamma rays; (4) the 



