Reproduction. 79 



RATE OF GROWTH IN MATURER PLANTS BEYOND THE 



SEEDLING STAGE. 



In general forestry practice the use of formulae is directed toward 

 estimating the amount of lumber in the trunk. The deduction of these 

 formulae is easier in the case of coniferous trees because of the continuous 

 growth of the chief shoot. Special problems demand formula 1 based 

 upon other data than the rate of growth of wood, e.g., in the business of 

 producing cork from Quercus suber. When forestry practice is directed 

 toward the culture of camphor trees, for example, in which the whole 

 bulk of the plant is to be used, the desideratum will be to determine the 

 rate of increase of weight. This is the case with guayule, since the whole 

 of the plant is used in the process of extraction of crude rubber. But 

 the rate of increase in weight can not be determined without introducing 

 the time element, so that we must first determine the rate of stem elonga- 

 tion in order to arrive at a general average of growth. But plants of the 

 same age are not invariably, or even quite usually, of the same weight, 

 since the relation of a plant to its environment results in more or in less 

 bushiness, in partial death and consequent loss of branches, in unusually 

 slow or rapid growth, or in total loss of plant by death. In estimating 

 the weight of shrub per unit of area for some future time it is evident that 

 all these factors are disturbing elements, the values of which may not be 

 easily determined. About the best we can do, therefore, is (i) to determine 

 the average rate of growth in length of stem, and (2) to determine the rate 

 of increase in weight for critical periods. The data indicate that there is a 

 period of relatively highest growth-rate, expressed in stem length or height, 

 and a period of relatively greatest increase in total weight of the plant. 



RATE OF GROWTH IN TERMS OF STEM-LENGTH. 



It has already been shown that the first season's growth results in 

 an average stem-length approximating 1 cm. A stem of this size has no 

 branches. During the second season's growth the stem may simply 

 lengthen, or it may also produce a number of short branches. This it 

 is more certain to do if the chief shoot produces an inflorescence. It 

 may otherwise merely elongate strictly for a number of years, resulting 

 in a very slow increase in weight, since the weight is affected chiefly by 

 the number of branches. At best the total weight assumed by a plant 

 in the first 7 to 10 years is small, seldom exceeding a few ounces. 



RATE OF GROWTH IN EARLIER YEARS AFTER GERMINATION. 



To determine precisely the age of a given seedling is more difficult 

 than would seem at first glance if it has been exposed to the weather for 

 more than a year. Furthermore, the rate of growth in many individuals 

 is so slow that the marks become well-nigh effaced, if not quite so. In 

 obtaining the following measurements, only plants which showed the 

 markings plainly enough to be seen clearly have been used. This has 

 very naturally thrown out those of very slow growth, in which the diffi- 

 culties are greatest, and thus the resulting average datum is probably 

 too great. By way of orientation two extreme cases may be cited. One 

 is a seedling of two seasons' growth, which germinated in 1907, making 



