Anatomy and Histology. 93 



though less than in endodermal cells with which they are in immediate 

 contact. Further development sees the collapse of the pericambium cells 

 (plate 22, fig. 6), and, as seen elsewhere, the primary stereome occurs in 

 the primary leptome just within the pericambium. 



The primary cortical cells outside of the endodermis are also capable 

 of secreting rubber. That they do so at all is contingent on the rate of 

 growth of the seedling. If this is rapid enough to remove the cortex 

 before drought sets in, no appreciable secretion will have occurred. If, 

 however, the rate of growth is lower, so that for the greater part of a year 

 the tissue in question remains functional, the inner cells at least may be 

 found densely filled with rubber. In the root, 5 mm. in diameter, of a 

 field seedling fully a year old, the following measurements (along a radius) 

 were made, from which an idea of the amount of primary cortex remaining 

 active may be had: Wood, 1.4 mm.; secondary cortex, 0.64 mm.; pri- 

 mary cortex, 0.15 mm.; cork, 0.27 mm. 



EARLY SECONDARY CHANGES IN THE STELE: (HADROME). 



With the completion of the primary hadrome plate there ensues a 

 centrifugal development of this tissue by the direct transformation of the 

 protogenic cells adjacent to the middle part of the plate. The increase of 

 hadrome extends along all radii except those lying near the plane of the 

 primary plate, but usually rather less rapidly toward the primary lep- 

 tome, so that in transverse section there appear two wings, so to speak, 

 of hadrome. This is protogenic, but is added to quite soon by the activity 

 of a cambium which first becomes apparent within and close to the pri- 

 mary leptome bundles (plate 22, fig. 10), and extends toward and finally 

 around the outer edges of the primary hadrome (plate 22, figs. 9 to 10). 



Up to this point in the development of the stele nothing exceptional 

 is seen. The only question which has been raised is in regard to the pre- 

 cise origin 1 of the earlier formed secondary hadrome elements, whether 

 this is by means of the cambium which arises on the inner surface of the 

 primary phloem, or is directly from protogenic elements lying adjacent to 

 the primary hadrome plate. The evidence from the material here under 

 discussion is that the latter is the case. 



Now, however, a behavior ensues which is somewhat unusual. Two 

 independent mestome strands of (at first) a single radial series of vessels 

 and a very small leptome strand arise, each, usually, in immediate con- 

 tact with the primary trachea (plate 22, figs. 9 to 1 1). The emergence of 

 a secondary root disturbs the exact position so that the earliest vessels 

 may be somewhat removed from the primary hadrome. A similar condi- 

 tion has been observed by me in Lamium amplexicaule , and by Petersen 2 

 in other Labiatae. 



It is necessary to note that although these mestome bundles are ver- 

 tically below the two lateral cotyledonary traces, we shall presently see 

 that they are independent of these and have no connection whatever with 

 them. 



I De Bary, Comparative Anatomy of the Phanerogams. 



I 1 have not seen this paper, but am informed by Dr. Holm. 



