188 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL X, NO. 6. 



THE MEMBRANACEOUS LEAVES. 



These leaves have a tubular sheath and a minute free blade, and they cover the subterranean 

 internodes of the stem. They arc perfectly glabrous and smooth, and contain no chlorophyll. 

 The epidermis is very thinwalled and consists of large cells on both faces, somewhat higher on 

 the dorsal than on the ventral face. A few layers of a homogeneous chlorenchyma surround the 

 veins, but between these the leaf consists only of the two epidermes. No collenchyma or 

 stereome was observed, and the mestome- rands are very thin, but surrounded by a parenchyma 

 sheath. In regard to the structure of the mestome-bundles we noticed that the hadrome was 

 better represented than the leptome. 



SUMMARY. 



These species of C«mmeUnaceae, which we have described in the foregoing pages, thus repre- 

 sent different types of biological interest: Commelina nudifiora, Tinantia, and Aneilema are 

 annual; all the others are perennial. Among the latter are some of which the habit resembles 

 that of the annual species, but which, nevertheless, are able to persist throughout the winter- 

 months, at least in the southern parts of this continent: Tradescantia Floridana and micrantha; 

 these maj' be designated as photophilous, their vegetative propagation being secured by axillary 

 buds developed wholly above ground. The other perennials (except Trad. Warssewicziana) are 

 scotophilous," in which the herbaceous stems die to the surface of the soil each year, and in which 

 the vegetative propagation takes place by means of buds developed upon rhizomes. In regard 

 to Tradescantia Warssewicziana, we feel uncertain about its manner of growth, even if we did 

 observe that the stem bears some buds underground. 



If we now examine and compare the external structure of the rhizomes and the roots with 

 that of the foliage, we do not notice any definite correlation between these. In our species of 

 Commelina the foliage is the same whether the species arc annual or perennial, and whether the 

 rhizomes are horizontally creeping with slender roots (('. hirtella) or very short and provided 

 with fleshy, thick roots (C. Virginica, etc.). In the genus Tradescantia we meet with species of 

 which the leaves are long and narrow and of which the rhizomes are either creeping and bearing 

 tuberous roots (T. jpinetorum) or very compact, with tuberous (7\ crassifolia) or more slender 

 roots ( T. Virginica, scopulorum). In Weldenia and T. Warszea'ieziana the leaves are fleshy and 

 arranged in a rosette; in the former the roots are thick and fusiform, in the latter they are 

 slender and much branched. In some of the other Tra<l>sc,i,ifi;e the habit becomes more like that 

 of Commelina, -especially iu T. Floridana and micrantha, with their short and broad leaves and 

 decumbent stems, or with the stems more ascending or erect as in T. eoininelinciides K. et S., 

 pulchella H. B. K., and disgrega Ki nth. The position of the leaves, as shown in the diagrams, 

 and the axillary shoots perforating the leaf-sheaths are also characteristic of certain members of 

 the family. 



While thus several variations exist in regard to the development of the foliage, stems, and 

 roots, we shall now extend the comparison to the structural peculiarities possessed by the same 

 organs as represented by these species. 



THE ROOTS. 



Simply nutritive roots were observed in Cminm Una nmlifftira, Tinantia, Aneilema, and 

 Tradescantia Florida na ; the slender roots of Weldenia belong to this same category. Nutritive 

 and at the same time conti-actile are characteristic of T. Warssewicziana and C. hirtella. 

 Nutritive and at the same time storage-roots were found in Tradiseantia crassifolia. In the 

 remaining species the roots showed a combination of contractile- and storage- roots. An 

 exodermis was observed in all the species, and it consists of two layers in Weldenia. Stere'ids 

 occur in Commelina nudiflora, Virginica, Tradescantia Fl<n i i<hni<i and crassifolia; they attain 

 the highest development in the last species. The cortical parenchyma is partly collapsed in 



"Goebel, K., Organographie der Pflanzen, Jena, 1900, p. 645. 



