298 EDITH BELLAMY SHREVE 



ly mask the true condition of the state of knowledge of cause and 

 effect. 



The investigations set forth in the following pages were car- 

 ried on at the Desert Laboratory at Tucson, Arizona. The 

 writer wishes to express, here her gratitude to Dr. D. T. Mac- 

 Dougal not only for the use of the facilities of the laboratory 

 but also for helpful suggestions concerning the work. 



Autonomic movements of an etiolated joint of a platyopuntia 

 were described by MacDougal in 1910, * and in 1912 2 he observed 

 an attenuated plantlet of Opuntia versicolor, that within a period 

 of three days showed movement in a vertical direction through 

 an arc of 180° and in a horizontal direction through an arc of 

 2 to 10°. During the last few years the writer has at various 

 times observed that joints or limbs of Opuntia versicolor, O.fusi- 

 caulis, 0. Toumeyi, and Carnegiea gigantea have changed their 

 positions from time to time. The work presented here was 

 undertaken with a view to finding the exact nature of these 

 movements and the cause of them. Opuntia versicolor was chosen 

 as the subject of the main part of the investigation on account 

 of the convenience of procuring large numbers of individuals. 

 This is a much-branched, semi-arborescent cactus, made up of 

 joints 1 to 4 cm. in diameter and 5 to 30 cm. in length. The 

 younger plants, 1 to 10 years old, have joints in all respects like 

 those of the same age on older plants. 



The plants used as material for the observations and experi- 

 ments retain the same designation by number or by letter 

 throughout the paper. They fall into three classes: (1) Nos. 

 1, 2, and 14, which were raised from seed in a green-house and 

 were 4 years old when used; their joints were more attenuated 

 than is usual with plants grown in the open but they were plump 

 and turgid. (2) Nos. 3 to 34 inclusive, excepting 14, which 

 were taken from the ground three months to a year before they 

 were used and were planted in pots. (3) Nos. A to G which 

 were growing in the open and left undisturbed. 



1 MacDougal, D. T., and Cannon, W. A., The conditions of parasitism in 

 plants. Carnegie Inst. Wash., 1910. 



2 From unpublished notes kindly furnished the writer by Dr. MacDougal. 



