Anatomy and Histology. 103 



2. On the contrary, in irrigated seedlings of very rapid growth the 

 vessels are much larger, though not quite so numerous, as in the plants 

 mentioned under (i) ; but the total amount of wood is considerably greater 

 relative to the diameter of the stem (plate 27, fig. 7). 



3. The terminal twig of a field plant of very large size, in which the 

 amount of growth in any twig was very small in one season, is contrasted 

 with an irrigated twig of rapid growth. The wood cylinders are equal in 

 diameter ; the vessels are somewhat larger in the secondary xylem of the 

 field plant. But in the protohadrome the vessels are larger in the irri- 

 gated plant (plate 27, figs. 4, 5). Both twigs of the same and last season's 

 growth. 



4. Two twigs of about the same diameter of wood cylinder, one a field 

 twig two years old, the other irrigated, one year old. The total number 

 of vessels is greater in the field plant, and there are more large and more 

 smaller vessels. In the protohadrome, however, the reverse as regards 

 size is true. But the number of vessels in either year's hadrome in the 

 field plant is probably the same as, or is less than, that in the irrigated 

 plant (plate 27, figs. 9, 10). 



5. On the contrary, in another irrigated stem 6.5 mm. in diameter, 

 the number and size of the vessels are enormously superior to the number 

 and size in a field plant (plate 27, figs. 2,3). 



6. The protohadrome in a field seedling of usual growth compared 

 with that of an irrigated plant, before secondary xylem has appeared in 

 either case. In the irrigated plant, in which growth is rapid, the elements 

 in question are much larger (plate 26, figs. 11, 13). 



7. The protohadrome in a peduncle, through which there is, relative 

 to its size, it can hardly be doubted, a very large transpiration stream, is 

 composed of very large elements (plate 26, fig. 12). 



8. In an etiolated seedling (plate 26, fig. 3), in which the size of the 

 stem remains small in relation to the total transpiring area, the size of the 

 conducting elements is greater, and their numbers scarcely less, than in a 

 field or irrigated seedling of approximately the same size of stem. 



9. In the tap-root of very rapidly grown seedlings the vessels are 

 much larger and the amount of mechanical tissue much less. 



These observations are in part antagonistic, in appearance at any 

 rate, to those of Cannon, and in part agree with them. They must there- 

 fore be harmonized among themselves as well as with Cannon's. In at- 

 tempting to cover all the cases with one explanation, we must not forget 

 that the problem indicated is a complex one, inasmuch as the ratios of 

 mechanical tissues in the two types enter into it. It will, however, suffice 

 to speak of the conducting elements alone at the present moment. 



In stems of guayule of a given diameter in field and irrigated plants, 

 the amount of wood is greater in the latter. In wood cylinders ' of equal 

 diameter the same holds true. This is due to (a) the smaller amount of 

 cortex in irrigated plants and (6) the narrower medullary rays. We may 

 assume that the growth in thickness of the stem is correlated with the 

 growth of the shoot above. In the same period, the total amount of con- 



1 Wood, medullary rays, and pith taken as a whole. 



