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



The Internal Structure of the Vegetative Organs. 



the roots. 



The secondary roots show the structure as follows: The epidermis is hairy and covers au 

 exodermis of large cells with the walls thin and distinctly folded. The cortex is differentiated 

 into three zones, a peripheral which consists of about four strata of relatively small cells, an inner 

 of about ten layers of large cells tilled with starch, and finally an endodermis. The endodermis 

 is thinwalled, showing the Casparyan spots very plainly: it did not contain starch. The 

 pericambium is thinwalled and continuous. The hadrome forms eight short rays, in which the 

 protohadrome vessels were observed to be mostly single or sometimes two arranged side In side. 

 Broad groups of leptome alternate with the hadromatic rays, and a few strata of thinwalled 

 conjunctive tissue surround the two central, reticulated and very wide vessels. 



THE STEM ABOVE GROUND. 



The internodes from the middle of the stem are cylindric, furrowed, and hairy. The hairs 

 are of the same kind as observed in 7. Virginica, lone- and pointed or short and clavate. 

 Epidermis is moderately thickened and covered by a thin, smooth cuticle. A thickwalled 

 collenchyma of about six layers, but in isolated groups, separate the epidermis from the cortical 

 parenchyma. The cortex is very thinwalled and consists of only four or five layers; it contained 

 chlorophyll. Inside the cortex is a closed sheath of stereome in one or two layers, but the cells 

 are rather thinwalled. Three almost concentric bands of mestome-bundles traverse the inner 

 part of the stem: the peripheral border directly on the stereome with their leptome, while the 

 two inner hands are located in the pith. 



THE STEM-LEAVES. 



The blade is glabrous and smooth, with a thin, but distinct cuticle. Epidermis consists of 

 large cells on both faces of the blade, and the outer cell-walls are slightly thickened on the dorsal: 

 stomata occur on both faces and the subsidiary cells are raised above the surrounding epidermis. 

 Prominent groups of thickwalled collenchyma cover the leptome-side of the larger mestome- 

 bundles; besides that an isolated group of this tissue occupies the outermost portion of the leaf- 

 margin. The chlorenchyma is poorly developed, and consists only of a few laj'ers of roundish 

 cells. The mestome-strands possess a thinwalled parenchyma-sheath, and show the same structure 

 as observed in T. Virginica. 



Tradescantia sp. — While the specimens of T. scopulorum, described above, were collected in 

 the mountains of Arizona, there is still another western member of the genus which inhabits the 

 alkaline plains of Colorado. This Tradescantia is by Mr. Rose included in his scopulorum, but 

 it appears to be distinct from this. It is a much coarser plant, with larger flowers and broader 

 leaves; the fore-leaves are destitute of blades, and the broad calyx-leaves and peduncles are very 

 hairy. The anatomical structure is somewhat different and may be described as follows: 



THE HOOTS. 



The secondary roots are fleshy, but slender and ramify but sparingly. Epidermis is thin- 

 walled and very hairy; it covers an exodermis of large, pentagonal cells, the walls of which are 

 thin ami prominently folded. The cortex consists of twelve compact layers of thinwalled 

 parenchyma, filled with starch. Endodermis and the pericambium are thinwalled, continuous. 

 There are six short, hadromatic rays alternating with six broad groups of leptome; the proto- 

 hadrome-vessels are very narrow, and are present in the number of two or three situated side by 

 side. The conjunctive tissue is thinwalled. and does not extend to the center of the root, which 

 is occupied bj r three wide, reticulated vessels. 



