54 T. Holm — Croomia pauciflor a. 



open on the lower face, thus representing some kind of pneu- 

 matic tissue. No stereome is developed, but there is a promi- 

 nent ridge of colorless tissue above and below the midrib and 

 the larger secondaries which becomes collenchymatic where it 

 borders on the epidermis. But this is the only mechanical tissue 

 in the leaf. The mestome-bundles are all collateral and single. 



Characteristic of Croomia pauciflora is, thus, the structure 

 of the mestome-bundles in the stem above ground, being lepto- 

 centric as well as in the rhizome, but simply collateral in the 

 axis of the inflorescence, in the peduncles and in the leaves. 

 The presence of similar leptocentric mestome-bundles is, 

 moreover, characteristic of the genus Boxburghia in accord- 

 ance with Mr. Lachner-Sandoval's investigations, cited above. 

 This peculiar structure, where the leptome is surrounded, more 

 or less completely, by the hadrome, is well known from various 

 other orders among the Monocotyledones, but mostly from the 

 rhizomes of these* ; it is known, also, from mestome-bundles 

 of certain Dicotyledones, which are located in the pith. In 

 other words, it seems as if this peculiar structure of the mestome- 

 bundles is principally observable in storage-organs and tissues : 

 rhizomes and pith. But in the Boxburghiacece this structure 

 is, furthermore, met with in the stem above ground instead of 

 only in the rhizome. 



Another peculiarity is the presence of stereome in the lep- 

 tome, sometimes as an isolated group in the larger mestome- 

 bundles or as a bridge in the smaller ones. The occurrence of 

 thick-walled cells in the leptome has been described by several 

 authors and defined as an abnormal thickening of the companion 

 cells or, in some. instances, of the sieve-tubes themselves instead 

 of pertaining to the adjoining strata of stereomatic tissue. 

 However it has been admitted that it is only occasionally that 

 such thick-walled cells in the leptome are clearly distinguish- 

 able from true stereome-cells. In Croomia, as far as the 

 writer has been able to ascertain, these cells were inseparable 

 from the supporting layers of stereome ; thus we have described 

 them as belonging to this tissue. 



It appears as if leptocentric mestome-bundles in stems above 

 ground are uncommon, at least among the Monocotyledones. 

 No such case has, so far, been recorded in the voluminous 

 literature dealing with Graminece and Cyperacem, and Mr. 

 Schulze does not mention the occurrence of such structure in 

 any of the Liliacece, Hwmodoracem, Hypoxidece or Vellozi- 

 acece, which he has studied and described in his interesting 

 paper on these orders.f 



Brookland, D. C., April, 1905. 



* Compare Chrysler, M. A. : The development of the central cylinder of 

 Aracece and Liliacece (Bot. Gazette, vol. xxxviii, p. 161, 1904). 

 fEngler's bot. Jahrb,, vol. xvii, p. 295, 1893. 



