138 T. Holm — Isopyrum biternatum. 



cycle contains actually two t}^pes of mechanical tissue. Five 

 collateral mestome-strands traverse the stele ; they contain 

 many vessels and cambium, beside leptome, and they are 

 separated from each other by several layers of thinwalied 

 parenchyma, which shows the same structure as the central 

 pith, being very thinwalied. The cambium is not confined to 

 the mestome-bundles, but occurs, also, between these as inter- 

 fascicular. 



The stem above ground. 



The stem is cylindric, glabrous and hollow. The cuticle is 

 thin and perfectly smooth, and epidermis (Ep. in fig. 3) is 

 somewhat thickwalled. No hypodermal collenchyma is devel- 

 oped, and the cortex consists only of three thinwalied strata, 

 bordering directly on the pericycle, which consists of about 

 three layers of thinwalied stereome (St. in fig. 3). Inside the 

 pericycle is a broad cylinder of thinwalied parenchyma with 

 seven collateral mestome-strands arranged in a single circular 

 band ; they contain very small-celled leptome, some strata of 

 cambium, and a Y-shaped group of wide vessels. It deserves 

 notice that the leptome does not border on the pericycle, but 

 is separated from this by layers of thinwalied parenchyma, 

 as may be seen from the figure (3). 



The leaf. 



The long petiole is cylindric, glabrous, and hollow. It has 

 a thin, smooth cuticle, and epidermis is slightly thickwalled. 

 The cortical parenchyma consists of about six layers of thin- 

 walied cells with chlorophyll, but there is no mechanical 

 tissue, neither collenchyma or stereome, and no endodermis. 

 There are six collateral mestome-strands, four large, and two 

 small ones, arranged in a circular band, surrounding a thin- 

 walied, partly hollow pith. This same structure we find, 

 furthermore, in the shorter petioles of the stem-leaves, in that 

 part of the petiole which is generally called " the character- 

 istic," just below the blade. In the petioles of the leaflets 

 the outline is hemicylindric, and there are only two large and 

 two small mestome-bundles, but otherwise the structnre is 

 identical. The small leaf-blades have a thin, smooth cuticle 

 above the thinwalied epidermis, of which the lateral cell-walls 

 are undulate ; the lumen of the epidermis cells is about the 

 same on both faces of the blade. The stomata are surrounded 

 by four to five ordinary epidermis cells ; they are level with 

 epidermis, and the air-chamber is shallow but wide. The 

 structure is bifacial, not only in regard to the distribution of 

 the stomata, which are confined to the dorsal face, but also in 

 regard to the chlorenchyma, which is here differentiated into 



