MEASUREMENTS OF GROWTH. 551 



irregular changes of temperature and light, and of the moisture of the air and soil, which 

 it is quite impossible for the observer to regulate and to overcome. The comparison 

 of the earlier observations shows that these circumstances have contributed in an 

 important degree to make the results not only of different observers, but even of the 

 same observer, conspicuously diiferent and mutually contradictory. 



' On these grounds I regarded it as the first problem to discover a method of 

 observation which allows of the measurement of any chosen and even slowly growing 

 small plant with sufficient accuracy, and where possible hourly. Suitable objects, 

 completely adapted for the purpose, are even in this case sufficiently difficult to 

 obtain, but still may be prepared by careful cultivation in pots : once obtained, 

 however, they may be submitted to observation in the laboratory under conditions 

 which can be varied as may be desired. 



' The mode of putting the question here arises, as in all experimental investi- 

 gations, from the consideration of pertinent phenomena already known, whence may 

 be drawn the conclusions which may possibly be expected. 



' If the object is to gain information as to the process of growth in length 

 of a part of a plant, in such a way that one obtains not only a connected idea of 

 it from the beginning to the end, but is also able to criticise the effects which 

 definite variations of temperature, illumination, and moisture induce, it is absolutely 

 necessary to measure the growths during short intervals of time — i. e. in one, two, or 

 three hours — and at the same time to know how the process of growth would have 

 proceeded if these external causes had remained constant. 



' That in the plant itself causes are active which, quite independently of the 

 variation in external conditions, sometimes accelerate the growth in length, sometimes 

 retard it, was moreover to be expected, and might be in part concluded from what 

 was hitherto known. Harting had already found that the stems of Hops grow at 

 first slowly and then more and more quickly, attain a maximum of rapidity, and then 

 again grow more and more slowly till growth at length ceases. Miinter also, 

 although his numerous observations were made at very varying temperatures, 

 recognised this fact, and expressed it in these words : — " In addition to the daily 

 rhythm, composed of exacerbation and remission, an increase, climax and diminution 

 (incrementum, acme, decremenium) of the intensity of growth take place. The lengths 

 produced rhythmically at first increase, ascend to a certain height, and then diminish 

 down to complete cessation." RauwenhoflF, so far, has most definitely expressed the 

 fact that in the course of a period of vegetation the growth of the stem first increases, 

 attains a maximum, and then slowly ceases.' 



I then pointed out, from the statements of Harting, Miinster and Rauwenhoff, 

 the existence of a grand period of growth, and demonstrated it for individual 

 transverse zones of stems and roots — a fact with which we have indeed already 

 become acquainted above. The irregular variations of growth, subsequendy 

 mentioned over and over again, although without satisfactory result, by different 

 observers, and apparently effected chiefly by internal changes, were likewise first 

 formally expressed by me in this treatise: — 'If then the grand curve of growth 

 affords an example of how the rapidity of growth of a part of a plant becomes equably 

 changed independently of external influences, or even in spite of them, it is to be 

 insisted, upon, on the other hand, that the powerful fluctuations in the growth in length 



