Anatomy and Histology. 95 



the growth of a mass of secondary stereome which arises within the pri- 

 mary stereome and is removed from it, in a root of 2 mm. diameter (plate 

 23, fig. 6), by about 35 microns toward the center. 



This secondary stereome strand is larger than the primary strand 

 and becomes an obvious structural feature. For this reason, and on ac- 

 count of its close proximity and its relative position to the primary canals, 

 it may very easily be mistaken for the primary strand. Its cells, however, 

 are larger, and it arises in connection with secondary leptome, and not in 

 relation to the proto-leptome. For this reason its position is more variable 

 than that of the primary strand and may suffer tangential displacement 

 (due to unequal development of the root) , as shown in plate 23, fig. 1 ; and 

 further, for less obvious reasons, the stereome may not occur at all. 



Other secondary stereome strands develop, if at all, always in con- 

 nection with the leptome, as stated for the stem by Fron and Francois 

 (1 90 1 ) and by Ross (1 908) . The particular mode of origin will be discussed 

 later. Each series is circular (plate 23, figs. 1 and 2), as all the members of 

 a series arise normally at the same time, though the series may be more or 

 less discontinuous, owing to unequal development as between the mem- 

 bers of the series. In seedlings grown rapidly under irrigation the amount 

 of stereome development is usually notably less than in field seedlings or 

 in others grown slowly. 



Secondary resin-canals (plate 22, fig. 13) arise within the secondary 

 leptome in close proximity to the cambium, and in the manner described 

 by Ross for the canals of the stem. They consist at first of two tangential 

 rows of cells scarcely distinguishable from the cambium from which they 

 arose, though quite early they may be recognized by their larger size and 

 the dense protoplasmic contents which at first show by their reactions 

 merely their protoplasmic nature. Ross (1908, p. 260), however, says 

 of these canal-cells: "Die den Kanal auskleidenden Zellen sind durch 

 dichtes Protoplasma ausgezeichnet, das sich mit Chlorzinkiod dunkel- 

 braungelb, mit Alkannin intensiv rot farbt, wahrend sonst das Leptom 

 hauptsachlich starkereichen Zellinhalt fuhrt." 



My own observations differ from those of Ross in that the cell con- 

 tents, when very young, do not react to alkanet as described by him. 

 Very soon after the two cell-rows begin to split away, minute globules of 

 a secretion begin to appear, and these indeed take on the intensive red 

 color of the reagent. This is considerably in advance of the same appear- 

 ances in the adjacent cortical cells. Preparations treated with alcohol to 

 dissolve out the resin or oil, which might be said to occur, show this very 

 clearly, and further treatment of the same preparations with benzole 

 shows that these intensively staining masses are dissolved out by that 

 agent, and, in the absence of evidence to the contrary, must be regarded 

 as rubber. In the secondary canal-cells, therefore, as in the wall-cells of 

 the primary canals of the root, occurs the earliest appearance of rubber, 

 the secretive activity extending progressively from them to the surround- 

 ing tissues, and more rapidly in the primary cortex. It is worthy of 

 especial note that rubber occurs in the wall-cells of canals which normally 

 contain, in the meatus, the resin characteristic of the guayule plant. This 

 point calls for discussion, which will follow later. 



