MINUTE STRUCTURE OF PLANT HYBRIDS. 263 



pureus. The leaflets of C. Laburnum are large, elliptic-ovate, silvery-hairy beneath, 

 and thin in texture, while those of C. Adami are elliptic, glabrous, less fleshy than in 

 C. pur pureus but inclining towards C. Laburnum in size. 



Leaf. — (a) Petiole base. 



In C. purpureus the epidermis is devoid of hairs. The cortex underneath is a tolerably 

 uniform cylinder of cells. A single crescentic bundle mass lies across the middle of the 

 petiole; beneath and above it are broken, irregular masses of sclerenchyma, whose elements 

 are small, measuring on the average 10 ll. Two small lateral bundles are also present. 



The phloem is 30 ll broad, and, in alcoholic material, of a very dark colour, due to the 

 amount of tannin material in its elements ; its sieve tubes are 3 to 4fi across. The 

 xylem is a single sickle-shaped band, whose main constituents are radially arranged rows 

 of spiral tracheids, each on the average 8 ll across, while single radial rows of small dense 

 cells lie between. 



In C. Laburnum the epidermis is abundantly clothed with the spindle-shaped hairs 

 already referred to. The subjacent cortex has its outer cells modified in colloid manner, 

 there being a single cell layer above which passes into two or three below ; internal to this 

 is a pretty broad zone of uniform cells, succeeded by one to three layers of larger, looser, 

 thin- walled cells, and this again passes into a broken sclerenchyma cylinder, made up of 

 a deeply concave, inferior mass, and of smaller isolated patches external to the smaller 

 bundles. The sclerenchyma elements average 15 //. across, though not a few may be 18 

 to 20 ll. The bundle system consists of one large inferior concave mass and of two to 

 three circular bundles, which almost touch each other. The entire bundle system, there- 

 fore, forms a broken ring and encloses a quantity of what we may term pith tissue. 



The phloem is 55 to 65 ll deep, and is of a pale reddish hue in alcoholic material from 

 the relatively small amount of tannin ; its sieve tubes are 6 m across. The xylem is an 

 almost continuous cylinder whose spiral tracheids are 20 /x across, and between these are 

 radial rows or irregular patches of cells. 



In C. Adami the epidermis has no hairs, as in the first, but otherwise the tissues 

 take largely after the latter parent. The subjacent cortex has one layer of colloid cells 

 above and two to three layers beneath ; these pass into large-celled, thin- walled tissue, and 

 that again into sclerenchyma disposed on the whole as in C. Laburnum, though more 

 sharply broken up into patches, thus inheriting the limited development seen in C. pur- 

 pureus. Each element averages about 12 ll across. The vascular system is arranged as 

 a broken cylinder, but the inferior part is deeper and stronger than the upper. 



The phloem in depth is about the same as in C. Laburnum, and the tint of it in 

 alcoholic material is brownish-red ; its largest sieve tubes are 4 to 4'5m across. The 

 xylem has spiral tracheids 14 to 15 n in diameter, which alternate with single or rarely 

 double rows or patches of cells. 



In C. Laburnum, along the sides of the petiole base are isolated patches of stone 

 cells, some being as much as 45 m across ; these are reproduced in C. Adami but are 

 wanting in C. purpureus. 



