MacDougaL — The Water-balance of Desert Plants . 83 
the 91 days ending February 3, 1911. This minimum was equivalent to 
one part in over 5,000, which was much in excess of the loss displayed by 
No. 4. The excessive loss is to be attributed to the greater development 
of green stems, five of which remained alive. The room was warmed to 
i8°C. for a few hours every day in February, but the rate of loss rose to 
only 0*16 g. daily. The rate was 0-2 g. daily during the first 20 days of 
March, o*2 g. daily during the next 60 days, and 0-32 g. daily in the period 
of 25 days ending May 1, when a number of rapidly elongating stems were 
being formed. (See lowermost figure in PI. X.) 
Tumamoca. 
Tumamoca is a relative of Ibervillea , native to the region about the 
Desert Laboratory, and having similar habits. The tuberous formations 
which retain the balance of food-material and water, however, are formed 
underneath the surface of the soil, and the outer layers are not so im- 
pervious to water. One of these structures taken from the soil and 
cleaned, November 2, 1910, weighed 125 g. Upon being placed on a 
suitable support in the laboratory it lost 1*3 g. daily during the first two 
days, or one part in 93, although care was taken not to damage the outer 
layers. This high rate continued, being 1*37 g. during the next five days, 
although it fell to 0-3 g. daily during the 86 days ending February 3, 1911. 
Its net weight at this time was but 90 g. and the minimum loss was one part 
in 400, which is greatly in excess of that of its relative Ibervillea , under the 
same conditions. (See uppermost figure in PI. X.) 
The total loss from this plant amounted to 258 g. or about 25-5 per cent 
of the original weight, which was greater than that shown by any other plant 
of this species. The excess is probably attributable to the greater number 
of green stems formed. 
Growth Activities of Desiccating Plants. 
In addition to the observations made on the growth or quiescence 
of plants with a water-balance during a period of depletion recorded in 
the preceding pages, the unpublished results of a study of the formation of 
stems from desiccating tubers of Dioscorea alata , which were made in the 
New York Botanical Garden, by the author in 1902, are of interest in 
connexion with the subject under discussion. 
Mature tubers produced by vines in the conservatories were placed 
in shallow glass dishes on tables within two or three metres from large 
windows in January of that year. The illumination did not differ greatly 
in intensity from that of the glass houses. Growth soon began and two 
or three buds awakened on each tuber, and the stems produced extended 
at an extremely slow rate during the following 20 months, with the 
result that an extension of 10 to 15 cm. was reached (PL IX, c). A 
