58 



A MONTANE RAIN-FOREST. 



fallen. Those on the sixth node will have been about one year old 

 at fall, and those which still adhere to the lower nodes may be of any 

 age up to two years. The lowest of the larger leaves are quite com- 

 monly covered with epiphyllous hepatics. 



A more exact measure of the growth of Pilea in terms of the size 

 of the plant was secured by making a computation of the relation which 

 was borne by the new to the old extent of leaf surface in two plants 

 that were under fortnightly observation and measurement, from the 

 middle of July to the middle of September. On the completion of the 

 measurements of these plants their green weight was secured and their 

 leaf area was determined by the method commonly used in transpira- 

 tion experiments. In the first plant eight leaves were in course of 

 growth from July to September, in the second plant twelve leaves. 

 The area of all the leaves on each plant in July was determined by 

 using the September area of all the mature leaves and an approximate 

 area for the leaves which had grown. This approximation was made by 

 considering each leaf as an ellipse, with the length and width in July 

 as the axes. The actual areas in September, the calculated areas in 

 July, and the amounts of growth are shown in table 15. The extent of 

 new leaf surface was 9.0 per cent that of the old in the first plant and 

 12.3 per cent in the second. In the lack of similar data for any other 

 rain-forest species or for the plants of any other region I am unable 

 to make any comparison of these figures with the performance of other 

 plants. 



Table 15. — Rate of leaf growth in Pilea nigrescens. 



In size and habit Pilea nigrescens closely resembles Pilea pumila of 

 the eastern United States. The plants of the former species which are 

 from one and a half to two years old are scarcely larger than the plants 

 of Pilea pumila which have grown from seed, germinating in late April 

 or early May, and have reached mature size in July or August. In 

 other words, the American species makes from six to eight times as 

 rapid development as the Jamaican species. 



There is no mathematically exact reciprocal relation between the 

 growth rate and average transpiration rate of the plants in which both 

 of these phenomena have been studied; indeed, it would be worth while 

 to seek such a relation only after the use of more exact methods of 

 growth measurement and more careful measurement of physical con- 

 ditions. There is every reason to believe, however, that the low rates 



