October 6, 1916] 



SCIENCE 



503 



ence phenomena in interpreting the water re- 

 lations of plants especially in growth, and 

 some conclusions founded on the assumption 

 that growing organs, like gelatine, would 

 show a maximum swelling in acidified solu- 

 tions are shown by our work to be mistaken 

 ones. 



During the course of some comprehensive 

 studies on growth now being carried out at the 

 Desert Laboratory it was deemed desirable to 

 follow the entire course of development of 

 shoots of Opuntia, and to make chemical 

 analyses at various stages. Growth depends 

 so largely upon the capacity for absorption 

 and retention of water that numerous meas- 

 urements of the swelling capacity of develop- 

 ing and mature members were made. 



The method consisted in cutting clean disks 

 12 mm. across from the flattened joints of 

 Opuntia. Three of these were arranged in the 

 bottom of a Stender dish and a triangle of thin 

 sheet glass arranged to rest its apices on the 

 three disks. The vertical swinging arm of an 

 auxograph 1 was now adjusted to a shallow 

 socket in the center of the glass triangle while 

 the pen was set at zero on the recording sheet. 

 Water or a solution being poured into the dish, 

 the course of the swelling was traced. 



That the amount of imbibition depended 

 mainly upon the presence of certain recog- 

 nizable substances and not upon properties of 

 the disks as masses of living material was 

 demonstrated by the fact that dried disks gave 

 proportionate differences equivalent to those 

 of living material. 



The average thickness of disks varied from 

 4 or 5 mm. in the case of young joints to 18 or 

 20 mm. in mature ones. The apical parts of 

 joints showed greater capacity for absorption 

 than the basal ones in the proportion of 21 or 

 22 to 16 or 17 per cent. Comparative tests 

 were finally based on disks taken from apical 

 regions. The capacity for absorbing water 

 was seen to increase up to maturity (about 1 

 year old) then to decrease as illustrated by 



i See MacDougal, D. T., ' ' Mechanism and Con- 

 ditions of Growth," Mem. N. Y. Bot. Garden, 6: 

 p. 14, 1916. 



the following set of tests with Opuntia 

 Uaheana made May 17-29, 1916. 



Opuntia blaktana 

 Swelling (distilled water) 



Young Mature 



24.30 500 



Old 



41.3# 



The amount of imbibition does not appear 

 as a continuous function of any one substance 

 or group of substances, the presence and 

 amount of which were estimated. This would 

 harmonize with the results of swelling mix- 

 tures of gelatine and agar described below. 

 The phenomena of proportionate swelling of 

 gelatine in water, acids, alkalies and salt solu- 

 tions have been mistakenly used hitherto in 

 attempts at explanation of the mechanism of 

 growth. It has been demonstrated by re- 

 peated tests that the tracts of growing cells 

 studied, as well as maturing or mature tissues, 

 do not swell more in acid than in distilled 

 water or alkali, as will be illustrated by the 

 ■following results taken at random from nu- 

 merous records obtained at Tucson. 



SWELLING OF DISKS OF OPUNTIA 



Young.. 

 Mature. 



Diat. Water 



Sodium Hydrate 



(Hundredth 



Normal) 



23.6% 



40% 



22.9% 

 52.1% 



Hydrochloric 



Acid (Hundreth 



Normal) 



16.4% 

 36.6% 



It is conclusively established that both 

 young and old tissues take up more water 

 when neutral or alkaline. Acidity therefore 

 in addition to retarding enzymatic action pre- 

 sumably including respiration would operate 

 to lessen growth by its effects in decreasing 

 imbibition by plant tissues. 



It being demonstrated that growing masses 

 of embryonic cells in plants and tracts of ma- 

 ture tissue show their greatest capacity for 

 the imbibition of water not in acidified but in 

 alkaline solutions, it was sought to find what 

 substance or mixture of substances would be- 

 have in a similar manner. The first inquiry was 

 made with agar which is composed of pentoses 

 presumably having some qualities identical 

 with those of the mucilages of the plant. Dried 

 cylinders and sheets of this material were first 

 subjected to the tests, being placed under the 

 auxograph after the manner in which disks of 

 living material were treated as described in a 



