92 Guayule. 



4 mm. in diameter, the reticulations were much less marked or absent. 

 The same condition is found in the definitive stem. 



This large size of the cylinder of tissue inclosed within the endoder- 

 mis is attained only under an abundant water-supply and other condi- 

 tions insuring rapid growth. Under such circumstances the structure of 

 the canal itself passes beyond the normal definitive stage, and the cells, 

 usually and normally eight in number in transverse section, may suffer 

 further divisions as shown in plate 22, fig. 5, to an extent sufficient, to- 

 gether with some displacement, to render it somewhat difficult to exactly 

 delimit the structure. In a root of this size, viz, 4 mm. in diameter, the 

 endodermis may still be readily recognizable (though irregular in charac- 

 ter, being in part of two rows of cells) by the starch-content or by the 

 Casparian spots, or both. The position of the endodermis is always clearly 

 shown, other signs failing, by the primary canals. 



The physiological changes in the endodermis are of particular inter- 

 est. Reference has been made to the variableness of the starch-content 

 of its component cells. When grown under irrigation the starch may be 

 seen in much larger plants than in those which have grown under normal 

 or field conditions. In these growth is less rapid and the extension of the 

 tissues correspondingly less marked. In these also the secretion of rubber 

 ensues earlier and is correlated with the occurrence of drought. In such 

 plants the cells of the endodermis, together with others to be noted below, 

 secrete rubber, so that, in a small seedling with a tap-root 2 mm. in diam- 

 eter, the cells of the endodermis will be found engorged with this sub- 

 stance. The cells of the canals are especially noteworthy in this respect. 

 By taking advantage of the effect of water upon the rate of secretion, it 

 may be shown that the secretion of rubber in the endodermis takes place 

 first in the cells of the resin-canals (plate 41, fig. 6). Thus, in the root, 

 4 mm. in diameter, of a seedling which grew with great rapidity, the canal 

 calls were half-filled with small droplets of secretion which reacted to al- 

 kanet. The specimen had previously been freed from alcohol-soluble sub- 

 stances, and there can, I think, be no doubt of the nature of the material 

 in question. 1 



The behavior of the pericambium in the region included between the 

 primary leptome and the endodermis differs from its behavior elsewhere. 

 One finds, in a root 1.2 mm. in diameter, that the pericambial cells have 

 enlarged radially and have in some cases undergone periclinal divisions 

 and the daughter-cells further radial divisions (plate 23, fig. 4). The peri- 

 clinal divisions suggest initial cork-divisions, but this is not the case, as 

 both the radial divisions and the further behavior of the cells show. With 

 a slight increase in thickening in the root, sufficient to attain 1.5 mm. in 

 diameter, the cell-walls are a little thickened and a rearrangement has 

 taken place. The cells have apparently been compressed between the pri- 

 mary leptome and the endodermis, and, under suitable conditions, as in 

 the specimen from which plate 22, fig. 5, was made, have secreted rubber, 



1 I have noticed that reaction to alkanet, which is the same in all the cells 

 at first, becomes in the canal-cells darker with time, the preparation having been 

 kept in darkness. I have attributed this, with some doubt, to the greater proto- 

 plasmic content of these cells. Great care was taken to extract very thoroughly 

 with absolute alcohol, and, in a part of my preparations, with caustic potash also. 



