66 REPORTS OF INVESTIGATIONS AND PROJECTS. 



species without the accompHshment of grafting. More striking results, how- 

 ever, were obtained by the making of combinations whereby species of widely 

 divergent morphological character were brought together. Thus, branches of 

 Optintia versicolor have been attached to P ouquieria splendens in such manner 

 that they derived a water-supply from the enforced host that kept them alive 

 for many months, while unattached branches withered and dried to dust. An 

 African Buphorhia derived a similar supply from the sahuaro ; Cissus digitata 

 from southern Mexico was successfully implanted in the storage-tissues of 

 Opiintia^ Bchinocactus, and Ccreus, thriving especially when cuttings were in- 

 serted in the fleshy joints of the flat-jointed opuntias. Furthermore, the 

 aerial roots of this form were capable of entering openings made in the epi- 

 dermis of the succulent host and penetrating to some slight depth, the exact 

 anatomical arrangement not yet having been determined. (Plate 3.) 



All the facts at hand seem to point to the suggestion that the essential con- 

 ditions underlying parasitism may be brought to light by a continuation of 

 these tests, especially if a chemical examination of the sap of the forms which 

 are successfully joined be made. The investigation will be pushed in this di- 

 rection. 



The determinations of volumetric changes occurring in succulents which 

 have been carried on by ]\Irs. V. M. Spalding, will also be brought into this 

 work, being of basal importance in certain features of the observations. 



The Course of the Vegetative Seasons in Southern Arizona. — In the colla- 

 tions of the known facts as to the general botanical features of North Ameri- 

 can deserts a detailed study has been made by Dr. MacDougal of the seasonal 

 activity of the vegetation of the Tucson region. Climatic conditions at the 

 Desert Laboratory are such as to allow of the definition of seasons as follows : 

 The winter wet season, extending from December to March; the arid fore- 

 summer, from March to July ; the humid midsummer, inclusive of July and 

 August; the arid after-summer, extending through September to the latter 

 part of November. The winter wet season witnesses the activity of a large 

 number of plants which differ but little from the vegetation of moist regions, 

 except in the fact that their seeds and shoots are capable of lying dormant 

 during long dry seasons. The earlier forms perfect their reproductive organs 

 about the first of February, after they have received 21,510 hour-degree 

 Fahrenheit units of exposure to temperatures above the freezing-point, or 

 more than five times as much as that which is required to bring the earlier 

 plants, such as Draba verna of New York, to bloom. The heat exposure 

 by the end of the wet winter season has now reached the total of 59'07i 

 units by April i, upon which the opuntias and the greater number of the cacti 

 and succulents have formed flowers and quickly proceed with the formation 

 of fruits. The beginning of the moist mid-summer witnesses the accumula- 

 tion of 160,539 units of heat, and with the accession of the rains a wide vari- 



