RATE OF GROWTH IN RAIN-FOREST PLANTS. 



57 



tions on it than on any of the other species. Plants of Pilea seldom 

 exceed a height of 50 cm. (20 inches), and maintain a smooth green 

 epidermis on their oldest stems. The leaves are opposite and com- 

 monly reach a mature size of 40 to 60 mm. in length, and are approxi- 

 mately half as broad as they are long. Two or three pairs of juvenile 

 leaves may frequently be found on the lowest nodes of plants which 

 have reached the usual size, such leaves being nearly orbicular and from 

 8 to 15 mm. in diameter. The inflorescences of Pilea are axillary, and 

 their existence and growth are found to have no retarding effect on the 

 growth rate of the leaves by which they are subtended. 



All plants of Pilea on which growth measurements were made in 

 1906 and in 1909 were situated on the floor of a Windward Ravine, 

 and were selected with a view to securing plants of average size and 

 full vigor. The maximum rates of elongation are given in table 13. 

 The entire series of rates of growth has been grouped according to the 

 length of the leaf at the beginning of each 

 interval of measurement the first group com- 

 prising the rates in leaves less than 10 mm. 

 in length, the second those from 10 to 20 

 mm. in length, and so on by 10 mm. inter- 

 vals to 60 mm. The averages of the groups 

 of rates give data for a curve of growth 

 rate, from which it is possible to learn the 

 mean rate of leaf growth at six successive 

 stages in elongation. The averages are 

 expressed in millimeters per day in table 14. 



From these rates of growth it is possible to determine the average 

 length of time required for a leaf to reach its mature size. Leaves which 

 attain a length of 40 mm. are 118 days old at maturity; those growing 

 to 50 mm. in length may be as old as 168 days, while those reaching 

 the maximum size at 60 mm. are probably 218 days old at full maturity 

 of size. It is possible that some of the leaves of maximum size make 

 a growth above the average rate throughout their development, and 

 thus reach the mature size in more than 118 days and less than 168. 

 It has been commonly found, however, that large leaves continue to 

 grow at a very slow rate, and it is on the basis of the growth rate of 

 such leaves that the computation of 218 days is made. 



The growth of a new pair of leaves begins at about the time thai 

 the next pair below them are half grown. The plants on which measure- 

 ments were made had from seven to twelve pairs of leaves. If the 

 leaves of these plants are assumed to have readied half their mature 

 size in sixty days, as would be the case if all leaves made the most 

 rapid growth, the age of the plants may be roughly estimated at from 

 fourteen to twenty-four months. Below the sixth or seventh aode 

 from the tip it is a common thing to find that some of the leaves have 



