100 Guayule. 



by secondary parenchyma rays (plate 25, fig. 7). The primary rays are 

 for the most part entirely closed, though two of these are suggested by 

 the topography of the pith, as above indicated. No more resin-canals 

 have appeared. The primary stereome bundles have extended inward 

 by the transformation of the primary leptome, and the primary resin- 

 canals are still present. Small secondary stereome strands are present 

 on the outside of several other bundles, as indicated in plate 25, fig. 10. 

 The endodermis is recognizable by its starch-content. The primary cortex 

 is much reduced, its tissue having been sacrificed to the development of 

 a thick cork, the original peridermal divisions of which take place in the 

 outermost cortical layer of cells (plate 23, fig. 8). 



The seedling in question (plate 25, fig. 7) was less than a year old, 

 probably four to six months. The epicotyl was 8 mm. long, with a few 

 small leaves, and was collected on July 24, 1908. 



An etiolated seedling (plate 25, fig. 9) of the same diameter, with an 

 epicotyl 2 cm. long and about three months old, shows a similar topog- 

 raphy, save quantitatively. There is a weaker and more irregular devel- 

 opment both of hadrome and of leptome. There is no additional stereome 

 beyond the four primary strands. The primary cortex is thicker and the 

 cork thinner. This seedling was supplied with abundant water and the 

 shade of a muslin cloth, with the effect of producing responses correlated 

 with a relative reduction of transpiration and to loss of water from the sur- 

 face of the stem. The greater leaf -area, together with a more slender axis, 

 results, however, in a greater transpiration stream relative to the diameter 

 of the wood cylinder, witn histological results to be noted beyond. 



An irrigated plant (plate 25, fig. 8) of slow growth, one which was 

 plentifully supplied with water, exposed to full illumination, but limited 

 in the spread of its roots, is very instructive in this connection. Under 

 these conditions we must assume a strong transpiration stream, at least 

 stronger materially than is usually the case in field plants. The specimen 

 had a diameter of 2.5 mm. and was not more than three months old, and 

 on this account alone was therefore a trifle larger and more advanced in 

 development than the preceding. In its cork development it resembles 

 the field plant, and has suffered the same reduction of the primary cortex. 

 In fact, in both cases one of the primary canals is just cut out by the peri- 

 derm. The deeper medullary rays communicate with the pith, indicating 

 secondary enlargement of the latter. The amount of wood as compared 

 with the field plant is much greater relative to age, but somewhat less 

 relative to radial measurement, and there is a relatively larger growth of 

 the secondary cortex. Most remarkable is the large and irregular devel- 

 opment of stereome. This irregularity is constantly associated with a 

 plentiful water-supply and is an expression of a general tangential dis- 

 placement of cortical tissues, as revealed by the later positions taken by 

 the primary resin-canals and the obliquity of the leptome masses, the 

 position of which predetermines that of the secondary stereome. 



Aside from the total quantity of hadrome, these three ecological 

 types present histological peculiarities which are related to the transpira- 

 tion stream. The number and size of the vessels in the field plant (plate 

 26, fig. 2) are scarcely inferior to those in the irrigated plants (plate 26, 



