MINUTE STRUCTURE OF PLANT HYBRIDS. 



213 



on each side by two scalariform vasa 50 to 70 /*, across, and dense, thick-walled tissue unites l 

 them. In front are scalariform or pitted tracheids, which project inwards to form the 2 

 greater extent of the xylem wedge. The innermost part of the wedge consists of a 

 transversely elongated mass of protoxylem tissue. 



In 2 the phloem patches (fig. 8) are 50 to 60 /x deep, the sieve-tubes are 25 to 28 /x across, 

 and the inner are rather larger than the outer, while the companion cells measure 7 to 9 //.. 

 The xylem is not so deeply wedge-shaped as in 3 ; the scalariform vasa are 75 to 85 fx across, 

 and are united by slightly indurated cells, in front of which are radially-elongated scalari- 

 form and pitted tracheids, while a small oval protoxylem patch completes the bundle. 



Leaf. — The petiole in parents and hybrid is divisible into a lower, flattened-out, and 

 concave region, in whose axil a cone-shaped bud develops. This is very closely protected in 

 Philesia by the concave petiole base bending up in knee-like manner round it ; in Lapageria, 

 however, and to a less extent in Philageria, the bud is well exposed. The upper petiolar 

 region, which extends beyond the level of the bud apex, is usually plano-convex, or a 

 groove mav traverse the flat face of it. 



These two regions are characterised by marked histological differences ; the matrix 

 cells of the lower part are only moderately thickened, but many of the upper are so 

 thickened as to become hard stone cells. 



The epidermis of the three agrees with that of the stem, except that in Philesia the 

 cuticular surface shows ridge-like stria? 1 ju or less in height, as compared with those of 

 Lapageria, which are 3 to 4 fi, and of Philageria, which are 1^ to 2 /a. That this structural 

 feature should be general in Lapageria, and only traceable over the leaf of Philesia, 

 affords strong evidence of their near relationship. 



Considerable variety exists in the distribution of the stone cells in the upper petiolar 

 region, but on the average few occur just beneath the epidermis in 1, though they are 

 massed as an indurated matrix round the central bundles. In 8 the entire circum- 

 vascular matrix is dotted over by stone cells, which, after staining and decolorising, are 

 very sharply differentiated from the unthickened cells. In 3 the distribution is evenly 

 intermediate, in some leaves examined, in others a massing towards the centre, as in 1, 

 predominates. 



Seven bundles, or rarely five, run through the petiole in 1 ; five bundles are usual in 

 f ; three, with at times two smaller ones, occur in 3. The size and number of sieve-tubes, 

 vasa, and tracheids are closely intermediate in the hybrid. 



The lamina may fairly be regarded as the most instructive part of the plant, for one can 

 scarcely desire to encounter greater diversity in two parents than is here shown in size, 

 form, consistence, and structure, while a more exact blending of these in the hybrid could 

 hardly be expected. 



On surface view * the upper epidermis of 1 shows cells of varying size (Plate III. fig. 6), 

 but with white wavy refractive walls. Though the majority are of a radiate type, it is 



*■ For preparations of the epidermis and other parts, as also for similar preparations of other species, the potash method 

 of maceration has proved invaluable (Proc. Brit. Assoc, Aberdeen, 1885), since it enables one to get clean and large areas 

 for examination.1 



Lapageria 

 rosea. 



2. Philageria 

 Veitchii. 



3. Philesia buxi- 



folia. 



