T. Holm — Ceanothus Amerieanus and ovatus. 529 



Mucilage-cells occur in the colorless tissue ; they are much 

 wider than the surrounding parenchyma-cells, but only a little 

 longer ; they contain a clear, colorless mucilage and are very 

 conspicuous in cross-sections. These mucilage-cells were in the 

 JRhamnacece first described by Guignard and Colin,* while the 

 occurrence of mucilage in epidermis of Rhamnus and other 

 genera of this family, but not in Ceanothus, has been discussed 

 by Blenk.f * 



The mid vein and the two lateral nerves from the base of 

 the leaf are quite thick on the dorsal face of the blade. They 

 are supported by collenchyma, as described above, but have 

 no stereome and no parenchyma-sheath ; the leptome and had- 

 rome is well developed, forming a broad, crescent-shaped 

 strand in transverse section with many rows of vessels. The 

 other veins are much thinner and are completely imbedded in 

 the green chlorenchyma ; in these the mestome is surrounded 

 by a colorless parenchyma-sheath, but with no support of 

 collenchyma ; the cross-section of these minute veins is elliptic 

 to almost orbicular. 



The petiole of the mature leaf is hemicylindric in transverse 

 section and hairy like the blade. There is no chlorenchyma, 

 and the very broad, single mestome-strand is directly surrounded 

 by a mass of colorless tissue in which the mucilage-cells are 

 very conspicuous (fig. 5.) A sheath of thick- walled collen- 

 chyma of about five layers is noticeable beneath epidermis. 



If we now examine the blades of the two opposite leaves of 

 the seedling, which succeed the cotyledons, we observe a simi- 

 lar structure, though with the following exceptions. They are 

 quite glabrous and epidermis is not thick-walled ; moreover, 

 the collenchyma is totally absent, besides that the colorless 

 tissue with the mucilage-cells is much less developed. It 

 might, also, be mentioned that the mediane and the two prom- 

 inent, lateral veins are surrounded by colorless parenchyma- 

 sheaths, which in the leaves of older specimens are restricted 

 to the thinner mestome-strands. 



The petiole shows the same tissues as observed in the mature 

 leaf, but the collenchyma is much less developed, representing 

 only a single layer beneath epidermis. Three wide mucilage- 

 cells were observed in the colorless tissue. 



The stern. — The hypocotyl of the seedling (H in fig. 1) is 

 cylindrical and almost glabrous. The cuticle is smooth, but 

 quite thick, and the outer cell-walls of epidermis are distinctly 

 thickened ; stomata and unicellular hairs were observed. 

 There is a narrow zone of cortical parenchyma of which the 

 innermost strata are partly collapsed and in which mucilage- 



* Sur la presence de reservoirs a gomme chez les Rhamnees. (Bull, de la 

 soc. Bot. d. France, vol. xlv, Paris, 1888, p. 325.) 



f Die durchsichtigen Punkte der Blatter in anatomischer und systematis- 

 cher Beziehung. Inaug. diss., Regensburg, 1884, p. 68. 



