64 CARNEGIE INSTITUTION OF WASHINGTON. 



The following conclusions are thought to be supported by this work: 



1. Shoots comparable in composition may be obtained at different times 

 of the year if similar roots are taken from cold storage and forced for equal 

 lengths of time. 



2. Light has little or no immediate effect on the growth-rate of young 

 shoots or seedlings, or on the height they attain before branching. Its effect 

 is chiefly through the food stored from previous photosynthesis. 



3. Light is an important factor in the production of new roots. It is 

 suggested that this may be a result of the change in carbohydrate gradient 

 due to photosynthesis. 



4. The three external factors of greatest importance in the growth-rate 

 of the young shoot are the temperature, the salt balance of the soil, and the 

 moisture-content of the soil. 



5. Temperature can not be considered a limiting factor in the narrowest 

 sense of the term, as an improvement in the salt balance of the soil induces 

 more rapid growth at the same temperatures. 



6. Increase in growth-rate is always found with increasing temperature 

 up to the highest which occurred in the experiments. 



7. The height attained by the stalk before branching is governed chiefly 

 by the temperature. If exhaustion of reserve food has an appreciable effect, 

 it is observed first in the production of stalks of smaller diameter, thus pre- 

 venting satisfactory comparison. 



8. The addition of sodium salts in low or medium concentration (0.001 to 

 0.2 molar) improves the salt balance of the soils and culture solutions used. 



9. With the culture solutions used magnesium salts begin to show an 

 injurious effect at a concentration of about 0.02 molar. 



10. The amino-acids present in the young stalk are favorable to high 

 imbibitional swelling of a protoplasm containing a large percentage of pento- 

 san. 



11. Temperatures favorable for commercial growth are limited on the one 

 side by reduced growth-rate and on the other by the early branching induced. 

 The color, texture, and slope of the soil are important in regulating soil-tem- 

 perature, which is of special importance when white asparagus is grown. 



PHYTOGEOGRAPHY AND ECOLOGY. 



Indirect Factors influencing the Vertical Distribution of Vegetation, 



by Forrest Shreve. 

 Data on the vertical distribution of vegetation secured during several years 

 of field work in different parts of southern Arizona have been collated and 

 prepared for publication. The nature of the physical conditions which 

 determine the vertical distribution of vegetation on desert mountains is now 

 well known. Throughout the mountains of southern Arizona there is a 

 close degree of correspondence between the sequence of the several types of 

 vegetation encountered in ascending from 2,000 to 10,000 feet. The com- 

 parison of distributional data from several mountains reveals some striking 

 differences in the absolute altitudes at which familiar transitions take place. 

 The precise altitudinal limits of the main types of vegetation are strongly 

 influenced by the departure from the normal altitudinal gradients of physical 

 conditions that are initiated by certain large topographic features. The 



