BAIRD: ANATOMY OF PLAT ANUS. 283 



Brown and Escombe's formula for finding the area of the 



stomatal openings per unit area of surface, A = 1 x w x 7 X 



number of stomata, we have .006567 X .00289 X -7854 X 124 

 (number of stomata per sq. mm.) = .01336 sq. mm. = area 

 of the stomatal openings per sq. mm. ; that is, when the stomata 

 stand open, the stomatal openings form about IV3 per cent of 

 the area of the lower leaf surface. 



In the leaves which I studied there was only one layer of 

 palisade cells, the usual layer just beneath the upper epidermis, 

 and not an upper and lower layer as Solereder states (p. 780). 



Fig. 7 is a surface view of a leaf embracing the epidermal, 

 the palisade, and the vascular bundle systems. It shows well 

 the free endings of the tracheids in the ground tissue of the 

 leaf. The greatest distance between the ultimate endings of 

 veins, or between an ultimate ending and the nearest vein, is 

 not more than .08 mm. Some of the tracheids of the leaf have 

 annular or scalariform thickenings (fig. 8, t) , and others have 

 transversely placed small oval bordered pits as in the tracheids 

 of the wood. The border parenchyma of the veins of the leaf 

 is composed of elongated cells with rather thick walls contain- 

 ing numerous pits (fig. 8, k) . The midrib and the larger veins 

 in the mature leaves have heavy strands of bast fibers, or these 

 united into a ring, surrounding the group of vascular bundles 

 (fig. 19, b.) Near the leaf blade the vascular bundles of the 

 petiole are arranged in either two or three rings, one above 

 the other, as shown in figure 20. Farther down the petiole the 

 bundles are arranged as shown in figure 21. Just above the 

 place where the petiole opens to include the bud the vascular 

 bundles become divided into groups which form a single ring 

 (fig. 23). The individual strands separate rather widely in 

 the part of the petiole which incloses the bud (fig. 24), and 

 enter the stem in this condition. Figures 25 to 28 show how 

 these groups of bundles approach the ring of vascular bundles 

 of the stem and occupy the leaf -trace gaps. 



Figure 2 represents a cross section of a very young leaf. No 

 cell walls have thickened excepting the walls of three or four 

 tracheids. Figure 3 shows a cross section of the midrib of a 

 somewhat older leaf. More differentiation of tissues is shown. 

 The cells have increased, not only in number, but also in size. 

 The walls of the cells which are to become bast fibers and the 



2 — Univ. Sci. Bull., Vol. IX, No. 21. 



