10 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



A preparation suitable for such work consists of a rooted joint 

 separated from a plant in the previous season, and fixed rigidly 

 in a setting of plaster in a metal disk with the roots imbedded in 

 soil or depending in water. Although water-culture seems anoma- 

 lous for cacti, yet preparations showed normal activity through- 

 out a growing season with the roots immersed in ordinary water 

 from a shallow well. 



Such joints offer the advantage of being mechanically firm 

 enough to support a lever counterweighted to give a bearing which 

 will allow the minute change in length to be recorded, and by 

 occasional adjustments of the instruments the course of growth 

 of a joint may be followed from the bud to approximate maturity, 

 a de\-eIopment which extends over two or three months. 



The use of the opuntias as material for the study of growth 

 might raise objections based upon the obscure and possibly com- 

 plex morphogeny of these plants. Such objections would have no 

 real weight. A joint supposedly consists of a number of inter- 

 nodes which mature as one in a developmental period of several 

 weeks. These structures are in effect thick disks of plasmatic 

 colloids enclosed in a distensible epidermal membrane, and tra- 

 versed by a fibro- vascular system of cellulose also capable of 

 extension. Carbohydrates are formed within this system by 

 photosynthesis and the catabolic or respiratory action is one by 

 which a high proportion of acids is accumulated at low tempera- 

 tures and in darkness. Water is drawn into the disk through 

 basal connections, and some is lost through the stomatal openings 

 in a characteristic manner. 



The opportunities for an analytical study of growth in such 

 plants were enhanced by the fact that the general course of water 

 loss had been determined by workers at the Desert Laboratory in 

 previously completed studies and that the daily and seasonal 

 variations in acidity had been followed in hundreds of instances in 

 certain species by Dr. H. M. Richards and by Dr. H. A. Spoehr, 

 while some measurements of the hydratation capacity or swelling 

 power had been made in 1914 by Mr. E. R. Long. 



Observations on the growth of a joint might therefore be carried 

 through several weeks and the daily and ontogenetic \ariations 

 correlated with acidity, turgidity, hydratation, and transpiration 

 conditions exhibited by controls and material previously examined. 



