38 THE RELATION OF DESERT PLANTS TO 



PLANT STUDIES. 

 INTRODUCTORY. 



As has been already pointed out, the main physical factor which 

 determines the nature of the vegetation on Tumamoc Hill is the 

 water relation. Except during the rainy seasons, this soil is far too dry 

 for most plants and only those forms can live here which are adapted 

 to dry soils and high evaporation rate. In the studies to be here recorded 

 an attempt was made to determine some facts in regard to the minimum 

 water supply with which desert plants can thrive. Studies of the 

 minimum water supply for germination of seeds were also made. 



Since it is next to impossible to make accurate measurements of 

 transpiration and water supply in the case of plants growing in the 

 ground, small plants were grown for the experiments in cylinders of 

 tinned sheet iron, perforated at the bottom to facilitate drainage. 

 Condensed-cream cans, holding from 250 to 300 cc., were found to 

 serve admirably for this purpose. Some cultures were made in Stender 

 dishes of the form used by microscopists for holding stains, but these 

 lacked drainage and were not as satisfactory as the tins. Only rain 

 water or distilled water was used for watering the cultures, since the 

 water from the supply tank contains much dissolved salt and the rapid 

 evaporation soon produced a sufficiently high concentration in the soil 

 to injure the plants. 



On account of the voracity of the desert animals insects, birds, 

 and small mammals it was soon found necessary to protect the cul- 

 tures by wire netting. A cage was therefore constructed for this 

 purpose about a meter long, 40 cm. wide, and 50 cm. high, raised about 

 40 cm. above the level of the ground. Ordinary mosquito screen of 

 about 3 mm. mesh was used for this purpose. This cage stood in the 

 open sunshine about 4 meters from the wall of the Laboratory and was 

 thus subjected to uniform weather conditions with plants growing in 

 the ground nearby. 



Several different plant forms were chosen for the work, some being 

 extreme xerophytes, others more mesophytic in their nature. The 

 fact that all work of this kind necessitates potted plants restricted the 

 choice of forms. It is almost impossible to lift from the soil and pot 

 mature specimens of those desert plants which live through the dry 

 seasons; their roots penetrate far into the soil, through openings 

 between the rock fragments, and can not be removed without injury. 



Of a number of the smaller forms with which transplanting was 

 attempted, only a few survived and produced new roots. One of these 

 was a small plant of Euphorbia capitellata. This is a form with small, 



