70 CARNEGIE INSTITUTION OF WASHINGTON. 



illustrated by the results of the observations on survivals of bulbs, 

 tubers, corms, etc., and various departures in habit and morphogenic 

 procedure have been induced. 



The following generahzations are included in the summary of results 

 as to the comparative values of various types of disseminules in sur- 

 vivals in new habitats and the receptivity of new habitats : 



1. Seeds and living plants of 139 species were used in the introduc- 

 tions into the four locations, and the behavior of these plants has been 

 followed during the 14-year period from 1906 to 1920. 



2. Several of the above species were put in more than one plantation, 

 bringing the total number of tests or introductions to 193. The actual 

 number of operations by the repetition of introductions brings the 

 total mmiber of operations to about 250. 



3. Only one survival resulted out of 10 introductions by bulbs or 

 corms, although many of these were duplicated, suggesting that this 

 form of the resting shoot is not well adapted to dissemination or estab- 

 lishment in new habitats without cultivation. 



4. Of the 31 introductions of plants which assume a resting form in 

 rhizomes and rootstocks, 8 survived. 



5. Of the 55 species transferred as plants with resting shoots, 20 sur- 

 vived in new locations, some in more than one. 



6. Of the 40 species which were carried as seeds, 12 survived in the 

 new locations. 



The following statements may be made as to the conditions of dis- 

 semination of plants and barriers to distribution : 



1. The dissemination of species toward lower levels takes place with 

 facility as the change is from low to higher temperatures, and the ac- 

 tual transportation of seeds or propagules would be aided by air-cur- 

 rents, gravity as making for earth and rock slides, and streams of water. 

 The isolated mountain and desert complexes, however, offer a range 

 of temperatures beyond the capacity for adaptation of all but a few 

 species. 



2. An action the reverse of this may be found in the distribution of 

 some of the opuntias; these are preeminently characteristic of the low- 

 lands and of the deserts, but a half-dozen species are represented in 

 the region of the oaks as high as 1,700 meters, while one species extends 

 beyond the oaks and is found among the pines in rocks at 2,300 meters. 

 It was discovered, however, that animals, probably rodents or rabbits, 

 operate a barrage against the movement of other opuntias up\vard on 

 the mountain slopes, as became evident by some experimental tests in 

 the Mohave Desert of California and by repeated introductions at the 

 Xeromontane plantation. 



3. It is rarely possible to ascribe the stoppage or restriction of a 

 species to the direct and simple action of a single physical agency. 

 The actual effect upon one species of defective humidity, low or high 



