14 BOTANICAL GAZETTE [july 



mam 



5 



structure. In Dianthera americana, on the other hand, the 

 axis seems never to produce a perfect mature type, but only 

 approaches it. The mature type is here first produced in a branch, 

 the rhizome. When once attained, however, it is persistent, as in 

 the case of the fern. 



Owing to the secondary development of vascular tissue, the 

 general appearance of the bundles, or meris teles, has changed 



■ 



considerably. Instead of being strictly collateral, as they were in 

 the earlier stages, there is a tendency for them to become con- 

 centric, the xylem and phloem being formed on the sides, and later 

 by the extension of the cambium, they are also formed on the inner 

 face of the meris teles. The cambium may even form a complete 

 ring, though this is not common in the seedling stages. 



The presence of this internal, and of course, inversely oriented 

 mestome, causes the node of an older seedling to have a rather 

 different appearance from that of the young seedling. On entering 

 the node (figs. 3-9), part of the vascular tissue on the sides of the 

 meristeles, sometimes of the phloem only, sometimes of both xylem 

 and phloem, passes around to the inner face, becoming inverted 

 during the passage. Part of the internal mestome of two of the 

 bundles turns into the transverse arm, and so connects with the 

 central bundle. Below the transverse arm there are three con- 

 centric rings of endodermis, one external to the complete ring of 

 normally oriented xylem and phloem, the second just within the 

 inversely oriented mestome. The third and innermost endodermis 

 surrounds the central cylinder. The normally oriented mestome 

 is separated from the inverted vascular tissue bv a laver. several 



abov 



in thickness, of closely packed parenchymatous cells. This 

 is continuous with the parenchyma of the individual meristeles, 

 1 and below the node. The middle endodermis is separated 

 from the innermost sheath by a layer of cells continuous with the 

 ground tissue of the internode, but much more closely packed 

 together. There is, however, still a considerable amount of air 



space 



ipiderm 



like those of the mature plant. As Holm has already accurately 

 described and figured these tissues, nothing more need be said of 



