650 



SCIENCE. 



[N. S. Vol. XII. No. 304. 



Lamson Scribner to be assistant botanist, with 

 instructions to devote himself to the study of 

 plant diseases. For a minor and secondary 

 place in the Division of Botany, this work, thus 

 begun, has grown into a separate division with 

 a large force of trained physiologists and pathol- 

 ogists. With this development in Washington, 

 there has been a corresponding growth in the 

 work in the experiment stations, while in many 

 of the agricultural colleges and larger universi- 

 ties courses of study in plant physiology and 

 pathology have been introduced into the botan- 

 ical departments. Where but a few years ago 

 so little was done in the study of plant diseases 

 that the term ' plant pathology ' was almost un- 

 known, good introductory courses in physiology 

 and pathology are now offered, and increasing 

 numbers of young men are familiarizing them- 

 selves with the scientific and practical aspects 

 of the problems involved. 



THE ANNUAL SHEDDING OF COTTONWOOD TWIGS. 



Just now (the middle of October) the Cot- 

 tonwood trees {Populus deltoides Marsh.) are 

 shedding their twigs, the ground beneath the 

 lai'ge trees being well littered over with fallen 

 twigs of all sizes. This curious phenomenon 

 has been noticed repeatedly, but still it appears 

 not to be generally known, even by botanists. 

 As the autumn advances the cortical tissues of 

 the bases of many of the twigs become so much 

 swollen as to produce bulbous enlargements. 

 At the same time there is a loosening of the 

 woody tissues in the same region, the result be- 

 ing that the woody cylinder is larger in diam- 

 eter at the base of each affected twig, and the 

 wood-wedges are separated from one another by 

 thicker medullary rays. There appeal's to be 

 a good deal of longitudinal tension exerted by 

 the swollen cortical tissues, the result being 

 that the woody tissues are pulled asunder, 

 showing a complete transverse fracture of the 

 whole of the woody cylinder. A breeze now 

 easily fractures the cortical tissues and the 

 twig drops to the ground. 



There is much apparent waste in this shed- 

 ding of these twigs, since they invariably have 

 large, well-formed terminal buds and generally 

 a good many lateral buds also. Among the 

 latter one often finds well-grown flower buds. 



These facts show that the twigs which are shed 

 are not the feeble and dying ones, but are 

 among the most vigorous and active on the 

 trees. It is an interesting fact that the Tam- 

 arisks {Tamarix sp.), which are held by some 

 botanists to be closely related to the Poplars, 

 shed their twigs by exactly the same device as 

 that described above. In the Tamarisks the 

 shedding of the twigs is a part of the annual 

 process of defoliation, their leaves being so 

 small that it appears to be less trouble and ex- 

 pense to drop twig and all than to separate 

 every individual leaf. Possibly in the Cotton- 

 woods, with their large leaves, we have a sur- 

 vival of the Tamarisk twig-shedding habit long 

 after its original significance has disappeared. 



THE IMMEDIATE EFFECT OF POLLEN. 



For a long time it has been known that in the 

 crossing of some plants the pollen seems to 

 produce an effect upon more than the embryo, 

 in other words, that not only the embryo but 

 other structures, also, show evidences of hy- 

 bridity. Focke named this phenomenon xenia 

 in a paper published nearly twenty years ago, 

 and this is the term now used by writers of 

 I)apers on this subject. The latest paper is an 

 exceedingly interesting one by H. J. Webber : 

 'Xenia, or the Immediate Effect of Pollen, in 

 Maize,' published as a bulletin (No. 22) of the 

 Division of Vegetable Physiology and Pathol- 

 ogy of the United States Department of Agri- 

 culture. In it an attempt is made to throw 

 light upon the real nature and meaning of the 

 phenomenon. Many experiments were made 

 by him to determine whether xenia actually 

 takes place in maize, with the result that its 

 occurrence is no longer to be doubted. It is 

 shown, moreover, that this immediate effect of 

 the pollen is limited to the endosperm of the 

 maize kernel. Thus where a change of color 

 occurs in the hybrid, this color is in the endo- 

 sperm cells, and furthermore, where the color 

 is in the pericarp (as in the variety known as 

 Eed Dent) no change in color takes place. 



The explanation suggested by DeVries and 

 Correns in papers published almost simulta- 

 neously in December, 1899, that xenia is the 

 result of double fecundation is adopted by Mr. 

 Webber without modification. In fact the same 



