MINUTE STRUCTURE OF PLANT HYBRIDS. 



233 



The bundle distribution to the water stomata is well illustrated in Plate VI. figs. 1-3, 

 and is as follows : — In 1 two lateral bundles run up the leaf-stalk on either side of the 

 median bundle, and terminate in the one to two lowest pairs of water stomata, though at 

 times a slender branch passes up which partly supplies the third. In 2 the first to the 

 seventh pairs in an average-sized leaf are wholly, and the eighth to the ninth pairs 

 partly, supplied by the lateral veins. In 3 the first to the thirteenth are thus supplied. 



The upper epidermis of 1 on surface view consists of large, straight, and uniformly 

 thick- walled cells, the walls being traversed by pore canals at pretty wide distances. 

 There are twenty to twenty-two cells in field of view under Zeiss' D with 4 ocular. No 

 transpiration stomata exist above, but towards the top of each serration one to two water 

 stomata (Plate VI. fig. 4) are set on the surface of a slight mamilla of epithem tissue. 

 In 3 the upper epidermal cells vary in size, some which surround stomatic clusters being 

 rather smaller than those of 1, others which lie between the stomata being greatly smaller. 

 They all show close-set pore canals, with knob"-like thickening of the walls between these. 

 There are fifty to sixty, exclusive of stomatic cells, in the field of view. Though Engler, 

 in his Monograph on Saxifrages (p. 13), says — " Die Spaltoffnungen treten sowohl auf der 

 Oberseite als auf der Unterseite der Blatter bei alien Arten auf, in der Kegel auf der 

 Unterseite zahlreicher," they are entirely absent on the upper surface in the "Geum" 

 section, though present on both surfaces in all the crusted Saxifrages already studied. 

 They are disposed in little island groups of from three to twelve, each group surrounded by 

 small epidermal cells. Surrounding the whole are the large cells already referred to, whose 

 contents are less dense than those just described. Each serration in 3 has one to two 

 water stomata set in a pocket-like depression of the epidermis, which may rise over them 

 in flap-like fashion on one side (fig. 6). Further, in all leaves of true S. Aizoon, a few of 

 the surrounding epidermal cells swell out into clear button-shaped knobs. In 2 the 

 epidermal cells are nearly the size of those in 1, twenty to twenty-five occurring under the 

 field of view ; but this is to be expected, if we bear in mind the relative size of cell and 

 leaf in each parent. The thickening of the walls is very evenly intermediate. 



Though a few of the earlier and smaller leaves of an annual rosette have no stomata, 

 they can readily be distinguished in most with a hand lens, running up> on either side of 

 the midrib. I have not counted the actual number on any leaf, but though there are 

 several hundred they are not nearly so abundant as in the " Aizoon " parent, while their 

 disposition in islands and their relation to surrounding cells are the same as in it. The 

 water stomata are very slightly sunk, or set on a flat surface, and a wave-like ridge of 

 the epidermis, rather than a pocket, exists on one side. It is specially interesting, how- 

 ever, to find that the epidermal knobs of 3 are reproduced, though in reduced degree, both 

 as to number and size (fig. 5). The gradually changing shape of the epidermal cells which 

 make up the serration tips in the three is a study in hybrid history which will well repay 

 careful tracing out, but without elaborate figures a description would be poor. 



The lower epidermis of 1 has two distinct kinds and sizes of cell somewhat as in the 

 upper epidermis of 3. Thus, there are large, wavy-walled, clear cells which collectively 



1. Saxifraga 



Geum. 



2. Saxifraga 



Andrewsii. 



3. Saxifraga 



Aizoon. 



