409 BOTANICAL GAZETTE [JUNE 
THE METHODS AND THE RESULTS. 
The number of ducts of the woody cylinder was determined in 
the following manner and under the following conditions: The 
branches selected for study were as nearly of the same size as possible, 
namely 0.6°™ in diameter; it did not appear feasible to get them 
all of the same age. All of the sections were made midway between 
the nodes. In counting the ducts an arbitrary area as a standard 
of comparison was taken as follows. A circle 14°™ in diameter was 
drawn on metric paper and cctants were struck off. By means of 
an Abbé camera lucida an image of the section was so thrown on the 
metric paper that the periphery of the woody cylinder and the two 
points delimiting any octant coincided. The standard octant had 
an area of 19.2499™ (14? Xo0.7845 +8). Since the magnification 
employed was 66 diameters, the area actually examined was 0.29% ™, 
or 29%4™™, 
The method of studying the ducts thus chosen gives the greatest 
prominence to the latest growth. This was desired for the reasons 
that the most active transfer of water probably takes place in the 
more peripheral ducts, and because I could be most sure of the recent 
history of the plants as regards irrigation. 
The term “irrigated” as used in this paper needs a word of 
explanation. I have been obliged to take whatever plants I might 
find that had received any water in addition to the rains. In certain 
instances, to be detailed below, the plants were growing by the side 
of an irrigating ditch and without question had water in abundance 
every day of the year. In other cases they were a greater or less 
distance from the ditch; and in yet other instances the ditch was 
filled with water for a portion only of the time, say once a month, 
during the dry season. Some of the forms received little water 
artificially, as, for example, Zizyphus, which is in the cactus garden 
at the University of Arizona and is not irrigated oftener than once 
or twice in a year. When the plants received a small amount of 
water I have called them “‘semi-irrigated.” The distinction between 
irrigated and semi-irrigated forms is thus purely arbitrary. 
The accompanying table presents a summary of the study. The 
numbers refer to the number of ducts in 29°¢™™ of stem cross section 
made under the conditions described above. 
