CISSUvS-OPUNTIA. 27 



CISSUS-OPUNTIA. 



The abundance of well-grown individuals of various species of flat and 

 cylindrical opuntias in the vicinity of the Desert Laboratory allowed much 

 use to be made of these forms in the experimental investigations, both as 

 possible parasites and as enforced hosts. The water-balance in these and 

 other succulents of the region is greatest in December-March and July- 

 August, and it was found advisable to set up the unions during these 

 periods, the greater number of survivals resulting from the experiments 

 started in February. 



February 1, 1908. — After a number of preliminary tests and examina- 

 tions, a number of slips of Cissus freshly cut from active stems were inserted 

 in joints of Opuntia discata and 0. blakeana. The slips were made by cut- 

 ting stems into lengths a few centimeters long, the basal portion extending 

 1 or 2 cm. below an internode which might be expected to furnish adven- 

 titious roots. The cavities in the joints of the opuntias were made by a 

 simple thrust of a glass or hardwood rod in the upper edge, extending 

 downward parallel to the surfaces, from which the cavities were generally 

 separated by a wall of greenish pulp at least 1 cm. in thickness. 



April 7 , 1908. — Actual growth in the preparations was exhibited by but 

 one of the slips, which had formed an aerial root from the lower exposed 

 internode 2 cm. in length. 



April 10, 1908. — Aerial roots were forming on a number of slips. A 

 number of new preparations were made. 



April 12, 1908. — An insertion on Opuntia discata displayed an opening 

 leaf -bud. 



Abril 17, 1908. — The bud noted above had enlarged in a normal man- 

 ner and two other insertions displayed similar activity. The insertions 

 made on April 10 had cast the young leaves borne at the time of the mak- 

 ing of the preparations. 



April 20, 1908. — The earlier preparations showed continued activity, 

 both in the development of leaves and aerial roots. Seven slips were 

 inserted in cylindrical stems of Opuntia versicolor (with yellowish flowers). 

 These remained alive a few weeks, but finally perished without making 

 any growth beyond some slight unfolding of the buds which were swelling 

 when the preparations were made. 



April 25, 1908. — Three of the original insertions still active, and three 

 more of the second set of insertions on Opuntia discata beginning growth 

 and forming aerial roots. 



April 28, 1908. — Practically all main buds active. The tips of aerial 

 roots of the preparations and also of plants which were growing naturally 

 were directed into punctures in tubers of sound potatoes and in joints of 

 Opuntia host. This test was repeated at various times, but in no instance 

 did an enlargement or branching of such roots occur, except in one case 

 noted below. 



May 12, 1908. — Nearly all of the insertions made in April were desiccat- 

 ing, as denoted by their shriveled appearance, while the slips put in the 

 hosts earlier were surviving in much better form. 



June 3, 1908. — Several new preparations were made, including two in 

 which etiolated branches of Opuntia were used. Some freshly-cut insertions 



