ECOLOGY — GEOLOGY. 343 



Rubber Plants, by H. M. Hall and Frances Long. 



The survey of the native rubber plants of western North America 

 has been brought to a close during the year. A portion of the work 

 was undertaken in cooperation with the University of California, and a 

 report on that phase of the investigation has been published. In this 

 report the presence of rubber in 16 species and varieties of Chryso- 

 thamnus and Haplopappus is announced. In addition to the coopera- 

 tive studies, special attention has been given to native latex-bearing 

 plants and to certain genera of composites. More than 250 species have 

 been investigated and the major number of these has been found to 

 contain rubber in some quantity. In the great majority the amount 

 is too small to be of importance, but in the case of 5 latex-bearing 

 species it is sufficient to indicate their probable commercial value. 



In certain species, samples of the plants and latex have been taken 

 at regular periods in order to determine the seasonal variation in rub- 

 ber content. Studies have also been made of the variation in rubber- 

 content with geographical distribution and ecological conditions, while 

 the amount present in different parts of the plant has been determined 

 in some cases. Experiments have also been made with reference to 

 methods of pollination, formation of seed, and vegetative reproduction 

 from harvested plants. Seeds of Euphorbia and Asclepias have been 

 planted in field plots at Tucson, Arizona, and Lincoln, Nebraska, and 

 methods of har\^esting have been worked out with reference to the 

 conservation of rubber-content. The plots also serve to indicate the 

 probable tonnage of the different species per acre, though they were 

 somewhat too small to make the results conclusive. 



GEOLOGY. 



Chamberlin, T. C, University of Chicago, Chicago, Illinois. Study of funda- 

 mental problems of geology. (For previous reports see Year Books Nos. 

 2-17.) 



The principal work of the year has been the preparation of a paper 

 whose purpose is to unify, revise, and extend the main results of the 

 studies that have thus far been undertaken under this grant. In the 

 course of this a special effort is made to bring out into clear definition 

 the dynamic principles and the concepts of force-distribution that 

 have come to be working guides in these studies. 



The first part of the paper consists of a review of the work previously 

 done, one purpose of which is to bring into their natural correlations 

 the results so far attained, while another purpose is to show the gradual 

 shift of dependence from static and material considerations to dynamic 

 considerations, together with the growth of rehance upon the latter as 

 the studies progressed. Early in the series of studies it was found indis- 

 pensable to take into serious account the earth's gravitative sphere of 



