330 EXPERIMENT STATION RECORD. 



ducted iu the garden during the spring and summer, while those upon the 

 herbaceous plants were carried on in the greenhouse during the winter and in 

 the garden during the summer. 



The conclusions of the author are as follows : 



" The four woody stems show no increase in mechanical strength or in the 

 amount or kind of mechanical tissue under the influence of longitudinal com- 

 pression. 



" In young herbaceous stems the development of mechanical strength in the 

 tissues is somewhat retarded by a longitudinal compression caused by compara- 

 tively heavy weights. 



" Neither light weights nor heavy weights have any appreciable effect upon 

 the growth and strength of herbaceous stems which have already formed a 

 cylinder of mechanical or woody tissue. 



" Continuous longitudinal compression causes no marked differences in the 

 size or form of any part of the stem which is subjected to the compression, 

 excepting of course mechanical changes which might be caused by excessive 

 compression." 



A bibliography is appended to the article. 



The delayed germination of seeds, L. H. Pammet. {Rpt. Brit. Assoc. Adv. 

 Sci., 1909, pp. 673, Gl'i ) . — A report is given of investigations on the germination 

 of weed seeds, a preliminary account having been noted elsewhere (E. S. R., 15, 

 p. 49) of earlier work, in which it was found that in general the effect of freez- 

 ing and thawing was to increase the percentage of germination, especially of 

 seeds with hard coats. 



The later investigations were conducted with the seeds of 130 species of 

 weeds, part of which were placed in paper packages and planted each month. 

 Another lot was stratified in sand and subjected to Iowa conditions. This 

 experiment was begun in the fall of 1905 and continued until the spring of 

 1909. The germination of the seeds was low during all the months, but in most 

 instances was better for the samples stratified iu sand than those kept in paper 

 packages. The fluctuation in germination is believed to be due to factors that 

 are not yet known. 



In the case of studies made on the vitality of soft maple seed, it was found 

 that these seed ordinarily soon lose their vitality but may be kept for a number 

 of weeks in the refrigerator without losing their germinating power. 



Relation of soil moisture to desert vegetation, B. B. Livingston {Bot. Oaz., 

 50 (1910), No. 4, pp. 241-256, dgms. //). — In continuation of a previous report 

 on the relation between vegetation and environmental conditions ( E. S. R., 22, 

 p. 325), the author presents data on the relation between soil moisture and 

 desert vegetation. Studies were made of soils occurring on the hill where the 

 Carnegie Desert Laboratory is situated, the plain below, and the slopes and 

 wash, the water-holding power of each being determined. 



During the driest months of the year the water-retaining capacity of the 

 different samples was 22.7, 16.1, 15.5, and S.4 per cent, respectively. Viewed 

 from the general plant activities, the different soil types respond to their pro- 

 portionate water-retaining capacities, the hill soil being fir.st, followed by that 

 of the plain, with the slope and wash soils in the order named. 



The results of the study emphasize the importance of considering the water- 

 holding power of soil in its relation to plant distribution. 



The fundamental causes of succession among plant associations, H. C. 

 CowLES {Rpt. Brit. Assoc. Adv. Sci., 1909, pp. 668-670).— The most important 

 factors that contribute to changes iu plant formations are held to be changes 

 in the humus content of soils and an increase or decrease in the amount of 

 shade. Other factors are plant invasions and topographical changes. Geolog- 



