i 



» 



I 



1907] HOLM—VEGETATIVE ORGANS OF RHEXIA 27 



The number of mestome strands varies in accordance with the thick- 



w 



ness of the tuber; the largest number observed was ten, representing 

 the growth of one season. 



The minor structure of these various tissues seems to be ver}^ uni- 

 form. The absence of ducts, stereids, and sclereids makes the 

 structure quite simple. The epidermis is sparingly hairy or per- 

 fectly smooth. The exodermis is thin -walled and consists of one 

 layer of cells pentagonal in cross-section, whose walls are not con- 

 tractile. The cortex is thin-walled throughout and frequently shows 

 radial collapsing; some of the cells contain druids of calcium-oxalate, 

 but no deposits of starch. The endodermis is thin-walled also and 

 shows the Casparyan spots very plainly; it surrounds a continuous peri- 

 cambium of a single layer, A few strata of more or less thick-walled 

 conjunctive tissue occur in the center of the stele and between the 

 leptome and hadrome. In the tuberous roots the cork is well repre- 

 sented {figs. II and 12), and the secondary cortex is compact, though 

 exceedingly thin-walled (C in fig. id). The secondary leptome {L in 

 fig. 10) shows a v^iy few cells, and the hadrome {H in fig. id) consists 

 of several wide vessels separated by thin-walled parenchymatic tissue 

 and by very broad rays of radially stretched parenchyma between the 

 mestome bundles. 



The section shown in fig. lo is from the tuberous root of the young 

 shoot shown in fig. 2, upon which a small bud is visible. The 

 root is in its second year and shows the enormous increase in propor- 

 tion to the very narrow primitive stele with the short rays of hadrome 

 {PH and H). The other section {fig. 13) is from an older and 



much thirkpr tnKf^ri^nc rnrkf \n wViiVli tTip number of vessels is laruer 



ome 



matic tissues attain a high development in 



of starch. It is to be noted also 



deposits 



primitive stele, and that the* central port 



stele has no pith, but only a few strata of conjunctive tissue. 



The stem 



body 



cylindric. The cuticle is thin, but wrinkled; the cells of the epider- 



