462 



SCIENCE 



[N. S. Vol. SLV. No. 1167 



The writer has during the past few years 

 propagated several thousand apple trees of 

 many different varieties on their own roots by 

 means of the common whip graft, but cutting 

 off the seedling nurse root after two seasons' 

 growth and replanting those trees which had 

 thrown out roots from the scion, thus estab- 

 lishing the variety on its own roots. Many of 

 these trees have been more or less troubled 

 with the crown gall and hairy root. It has 

 been observed that there is a tendency for a 

 given variety to have only a single form of the 

 disease. Thus the Jewett apple shows usually 

 if not always the hard form of the gall, the 

 Red Astrachan the simple form of the hairy 

 root and the Oldenburg the woolly knot form 

 with many soft fleshy root growths. Other 

 varieties show the broom root form and still 

 others often the aerial form. 



In the ordinary method of propagation of 

 apple trees the root systems are of seedling 

 origin and from a pomological viewpoint the 

 root system of every tree is a different variety. 

 May not this be the reason for the various 

 forms of crown gall and hairy root? 



Some varieties on their own roots seem to 

 be largely if not entirely immune to this dis- 

 ease. If this proves to be really the case, here 

 may lie the solution of the problem of the pre- 

 vention of crown gall. If a resistant variety 

 is selected as the root variety, and the variety 

 desired propagated on it, trees immune to the 

 disease may presunaably be secured. Prob- 

 ably the economic advantage would warrant 

 the extra effort necessary to propagate such 

 trees, only under conditions where the crown 

 gall was especially troublesome. 



There are other root diseases which are in- 

 jurious, especially through the southern part 

 of the apple belt, that might possibly be 

 avoided in a similar fashion. 



J. K. Shaw 



Massachusetts Experiment Station 



when a force is a force 



Eeferring to the perennial discussion of the 

 meaning of force and of the law of action and 

 reaction, lately revived in the pages of Sci- 



ence,^ I venture to suggest that the essential 

 point of the alleged difficulties which have been 

 raised is covered by the following simple prop- 

 ositions : 



1. A force is a push or a pull exerted upon a 

 body (portion of matter) by another body. 



2. Whenever a body A pushes or pulls a 

 body B, then at the same time B pushes or 

 pulls A equally in the opposite direction. Such 

 a pair of forces is an " action and reaction." 

 An action-reaction pair concerns two bodies 

 and only two. 



3. The two forces of an action-reaction pair 

 never balance each other; a force acting on A 

 can not balance a force acting on B. 



4. To balance a force acting on B, another 

 force must be applied to B. 



One who keeps these simple facts in mind 

 will, I believe, find it easy to decide whether 

 an alleged force is really a force in the mean- 

 ing of the ^Newtonian laws. He will also see 

 that there is no contradiction between the 

 statement that forces always occur in action- 

 reaction pairs and the statement that forces 

 are often unbalanced. L. M. Hoskins 



STANrOED Univebsitt, 

 March 22, 1917 



SCIENTIFIC BOOKS 



The Potato. By Arthur W. Gilbert, assisted 



by MoRTiER r. Barrus and Daniel Dean. 



New York, The Macmillan Co., 1917. Pp. 



i-xii and 1-318, PL XVI. 



The author states in his preface that the 

 book is intended to give brief and practical 

 suggestions on the growing, breeding and mar- 

 keting of potatoes, and the subject-matter 

 amply substantiates the statement. This pub- 

 lication, in addition to being up-to-date in its 

 cultural directions, devotes considerably more 

 attention to the subject of potato breeding 

 than any of our preceding American treatises 

 on the potato. Conveniently arranged statis- 

 tical data are presented in Chapter I. under 

 the caption of Acreage, Distribution, Pro- 

 duction and Valuation. Chapter III. em- 



1 See articles by Gordon S. Fulcher (November 

 24, 1916), and Andrew H. Patterson (March 16, 

 1917). Mr. Fulcher 's discussion seems to me to be 

 entirely sound. 



