26 BOTANICAL GAZETTE [july 



protection. He calls attention to its common occurrence in lianes 

 and in creeping plants, and says that the protection counter- 

 balances the danger to the phloem due to the great length and 



weakness of the stem. 



Col (2), on the other hand, presents the following hypothesis 

 (translated from p. 275): "The histological structure of the con- 

 ductive tissues does not permit a sufficient condensation to form a 

 single circle. The wood becomes condensed, more easily than the 

 phloem, into a small bundle; without doubt on account of the 

 fluidity of the ascending sap, and of the easy passage of liquids 

 from one vessel to another. For converse reasons, the phloem is 

 less capable of becoming condensed." When two bundles come 

 together, therefore, the phloem passes around the sides of the wood 

 to the inner face, or it may even become medullary. While it is 

 undoubtedly true that the phloem is better protected in its internal 

 position, as suggested by Vesque, yet from my observations on 

 Dianthera americana, it would seem that the hypothesis of Col 

 certainly holds in that plant. 



It may be that, in this case, the internal vascular tissue is cor- 

 related with the lack of a complete ring of vascular tissue. The 

 phloem, not finding sufficient room on the outer face of the indi- 

 vidual bundles, becomes crowded around to the sides, or even to 

 the inner face of the bundles. The new vascular tissue descending 

 from the uppermost leaves does not find room for its insertion, so 

 part of it at least passes inward to form the central bundle. After 

 it has begun its development imbedded in the pith, a sheath finally 

 differentiates to separate it from the air-containing tissue of the 

 much modified mature pith. 



As to the cause of the astelic conditions found in this and other 

 species of Dianthera, very little can be said. It is probably cor- 

 related, however, with the aquatic habitat. The large amount of 

 air space undoubtedly is to be correlated with the aquatic habitat. 

 If we agree that the endodermis is not a morphological boundary, 

 but a physiological layer separating the vascular tissue from the 

 "air-containing lacunar system, " as claimed by Strasburger, 

 then we have a simple, plausible explanation of this phenomenon. 



More comparative work on this and other species of Dianthera 



