102 Guayule. 



living part. Two series of secondary canals have arisen in the hypocotyl 

 of the size under consideration, whether the growth has been rapid or 

 slow under irrigation (plate 29, figs. 3, 4), or slow in the field (plate 28, 

 fig. 3) ; but the total number of canals is greater in the irrigated plants, 

 as would be expected in view of the more numerous wood plates. The 

 radial depth of the cork has not increased in any appreciable amount in 

 the field plant, but is more uniform than in a rapidly growing plant, in 

 which it is relatively much thinner (plate 29, fig. 3). 



It is of interest to extend our comparison to rapidly and slowly grow- 

 ing irrigated plants. The chief point of difference is seen in the much 

 greater tangential development of sieve-tissue, and, later, of stereome, 

 relatively to the size of the plant in slowly growing specimens (plate 29, 

 fig. 4) . This statement may be extended also to the mechanical elements 

 of the wood, in which the libriform cells are of smaller diameter, have 

 smaller lumina and are more cylindrical, implying a greater amount of 

 intercellular cementing substances. The vessels too are of smaller diame- 

 ter, and, though this is compensated for by their greater numbers, the 

 capacity of the vessels in the more rapidly grown plant is considerably 

 greater (plate 27, figs. 6, 7). The phloem presents analogous differences, 

 having in the slowly growing plant a structure denser and much more 

 extended tangentially than in the rapidly grown plant, and in this, as in 

 the character of the wood, resembling more closely the field plant (plate 

 28, fig. 3). A still further difference, of more fundamental character 

 morphologically, is the development, in slowly growing irrigated plants, 

 of stereids in the pith (plate 29, fig. 4). So far as I have been able to 

 observe, the stereids occur under no other condition in the hypocotyl, 

 though, as will be shown, it occurs normally in the pith in the definitive 

 stem (plate 29, figs. 5, 6). 



The observations on the structure of the wood in the seedlings studied, 

 regarding especially the water-carrying elements, are of peculiar interest 

 as they stand in relation to those of Cannon (1905), who studied compar- 

 atively irrigated and non-irrigated desert woody plants of eight species. 

 His general conclusions, undoubtedly supported by his observations, are 

 that "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," but he says at the same 

 time that this "is an unexpected condition." Further, "irrigated plants 

 organize each year a larger amount of wood which contains a relatively 

 large amount of non-conductive tissue so that it happens that non-irri- 

 gated and older stems have more vessels than irrigated and younger" of 

 the same diameter. 



For the reason that I found, to my surprise also, that some of my 

 observations coincide with Cannon's, I venture to cite certain concrete 

 instances, and state these, together with those already presented, in brief 

 fashion, by way of instituting a comparison of our results: 



1 . In field plants (the seedlings above studied) the vessels are as large 

 as in irrigated plants of slow growth, or larger, and are slightly more 

 numerous. The stems are of nearly equal diameter (plate 26, figs. 2,4; 

 plate 27, figs. 6,8). 



