406 BOTANICAL GAZETTE [JUNE 
of the University of Arizona in the cactus plantation. It receives 
water by direct application about twice a year. The nearest irrigating 
ditch is across the drive, about 13™ distant. It should be explained 
that the drive is so constructed with reference to the underlying 
calliche that presumably no water seeps across. 
GENERAL CONCLUSIONS. 
Although, much to my regret, the amount of water given to the 
irrigated plants is not known with any degree of exactness in any case, 
there can be no mistaking the fact that branches of irrigated plants 
(even if “‘semi-irrigated” only) are poorer in conductive tissue than 
branches of the same diameter of non-irrigated plants. This is an 
unexpected condition and of especial interest in view of the small 
development of the water-conducting elements in the non-irrigated 
forms of the Egyptian-Arabian deserts as given by VoLKENs. It 
merits further investigation. 
Without doubt the irrigated plants have a greater absolute trans- 
piration and organize each year a larger amount of wood than the 
non-irrigated plants of the same age; but the composition of the 
wood is different in the two instances. The irrigated plants con- 
struct a relatively large amount of non-conductive tissue each year 
(as compared with the amount of the other wood elements formed), 
while the reverse is true of the non-irrigated plants. So that it 
happens, in stems of equal diameter but not of the same age, that 
the non-irrigated and older stems have more vessels than the irri- 
gated and younger. One other characteristic of non-irrigated stems 
was also noted, namely, their ducts were usually or frequently of 
greater diameter. 
We find, therefore, a quantitative and a qualitative difference in 
the structure of irrigated and non-irrigated stems, and from what 
has been stated it appears that any adequate account of the causes 
which induce these structural peculiarities must separate the pro- 
cesses which may be associated with the formation of the woody 
cylinder as a whole from those which may be connected with the 
organization of that part of the wood that is primarily to conduct 
water. 
I shall not presume at present to offer any hypothesis to explain 
