140 AMERICAN JOURNAL OF BOTANY [Vol. 8 
■ 
full leaf and when growth has diminished, its resistance to injury will be 
greater and the liability of poisoning by it less. 
The malignancy of the plant may also be considered in relation to its 
visibility or conspicuity. From this standpoint the virulency of the plant 
would be indirectly proportional to its conspicuity. The plant is least 
conspicuous when it is not in leaf, more conspicuous in the spring when the 
leaves and flowers are expanding, still more easily recognized when in full 
leaf, and most likely to be observed when its leaves have assumed their 
bright autumnal colors. 
The virulency of the plant may be summarized according to its toxic 
portions, the virulency of the resinous sap, the turgescence and ease of 
fracture of its parts, the conditions of light and atmospheric humidity, and 
its conspicuity. The liability of poisoning, then, by R. diversiloba tissues 
decreases as follows: immature leaves and flower parts (except anthers and 
pollen), mature leaves, green stems, young roots, woody stems, and woody 
roots. According to the amount of poison in the plant, however, virulency 
would be greatest during the period of full leaf. This factor gives way 
before the far greater balance of factors just mentioned. 
This theoretical consideration of the liability to Rhus poisoning from a 
botanical point of view has its counterpart in clinical statistics. The 
latter lend analogous evidence to the conclusion that spring has the greatest 
number of cases, that a sudden decrease in cases occurs during the time of 
the autumn tints, and the least number of cases takes place during the 
dormancy of the plant from November until February. It should be 
noted, also, that in 1915 the greatest number of cases among Berkeley 
students (table i) was in March, previous to the opening of flowers about 
April 4, and that in 1916 the maximum number of cases occurred during 
February previous to the maximum flowering period (March 22 to May i) 
of that year. This evidence contradicts the belief prevalent among many 
people that the plant is most malignant during its flowering period. Some 
of the opinions expressed in medical literature in regard to malignancy are 
as follows : most cases usually in spring (Busey, 9) ; most noxious at the 
period of efflorescence (Yandell, 63); most cases in summer and autumn 
(Park, 46); greatest activity during the flowering season, from May to 
October (Philadelphia; Blackwood, 5); most virulent July i to September i 
(Hubbard, 29); worst in December when buds are coming out and in May 
when leaves fall (California; Baldwin, 2); poisonous at all seasons of the 
year (Philadelphia; Beringer, 3); most poisonous when in bloom (Davis, 
1897); cases most prevalent at the season of the year when the foliage is 
beginning to show itself (Cantrell, 10); most poisonous when in bloom 
(Harriman, 1898); especially virulent just as the buds come in the spring 
(Thudichum, 56); more active (in New York) during the summer months, 
the last two months of spring, and the two first months of autumn (Hadden, 
22). 
