MINUTE STRUCTURE OF PLANT HYBRIDS. 



221 



is too common to need description. As every one is aware, many varieties of the latter are 

 in cultivation, and the strain which furnished pollen is not definitely known. Equally from 

 naked-eye and microscopic examination I regard it as one which had bright green vegetative 

 parts, and a corolla white on first opening but gradually becoming crimson with maturity. 

 Such a strain existed in the Edinburgh Garden, and flowered during the past summer. 



Accepting this as probable, it may be noted that in D. alpinus the flowering shoot 

 produces four to five pairs of vegetative leaves beneath the inflorescence; that of D. barbatus, 

 nine to ten; and that of the hybrid, six to seven. The calyx is claret-coloured in the first, 

 and one, or it may be all, of the sepals show a red tip. That of D. barbatus is green, while 

 the hybrid shows an intermediate tint with red tip to one or all of the sepals. 



I need not describe the naked-eye appearances further than to point out a peculiar 

 periodic colouring of the corolla referred to above. When the petals of D. alpinus first 

 push out from the calyx their outer surface is white, and the inner is pale pink. On 

 full expansion they are rose pink with crimson eye, while before withering they assume a 

 deep purple-crimson hue, this happening about nine days after expansion. In the strain of 

 D. barbatus noted above, each flower from the bud state till two days after expansion is 

 white, then it assumes a gradually increasing pink tinge till the sixth day, when it is rose 

 pink, it then deepens till the twelfth day, and before withering is of a crimson colour. In 

 the hybrid an intermediate series of changes are passed through, the final colour being 

 paler than that of the seed parent. 



Stem Structure. — The mature stems of the second year from which the leaves have 

 withered are in proportion of 1 : 1^ or 2 : 3. The stem epidermis of 1 (Plate IV. fig. 7) persists 

 as a layer round an outer cortex of 3 to 4 zones of cells, of which the most external are largest : 

 within this is a cork cambium, on either side of which six to eight layers of cork and 

 phelloderm have formed, the latter adding to the depth of the internal four-zoned cortex 

 which surrounds the phloem. In D. barbatus (fig. 9), the epidermis and three to four 

 external layers of outer cortex cells easily separate from one to three internal layers, which 

 persist and surround a sclerenchyma sheath four to five cell-rows deep, within which a faint 

 line of cork cambium can be traced. The internal cortex consists of seven to eight zones, 

 whose elements are largest in the middle. In the hybrid (fig. 8) there is a distinct separation 

 tendency in the epidermis, and there are one to two cortical cell layers ; internal to these are 

 two to three layers lying against a sclerenchyma sheath that is two to three zones deep, 

 and the elements of which are half as much thickened as those of the last. The cork 

 cambium is evident, and three to four derived layers lie external and internal to it. The 

 inner cortex consists of five to six cell-rows whose units are on the average intermediate 

 between those of the parent. 



The phloem of the three closely agrees in size of the elements, though it differs greatly 

 in total amount in the three forms. 



In 1 the xylem vasa are few in number, but sharply isolated ; in 3 they are numerous, 

 and often in groups of two or three ; in the hybrid they are intermediate in number, but 

 seldom grouped. 



1. Dianthus 



alpinus. 



2. Dianthus 



Grievei. 

 8. Dianthus 

 barbatus. 



