MINUTE STRUCTURE OP PLANT HYBRIDS. 265 



C. purpureus, upper epidermis = 27 to 30 stomata. 



„ lower „ = 30 ,, 



C. Adami, upper epidermis = 12 to 14 „ 



„ lower „ = 17 to 20 „ 



C. Laburnum, upper epidermis = (except a few along base of midrib). 



„ lower „ = 40 



The above gives us an average on upper and lower areas collectively of 58 to 60 in 

 C. purpureus, of 31 to 33 in C. Adami, and 41 to 42 in C. Laburnum. 



The epidermal cell-nuclei of fresh or carefully hardened leaves agree as above noted 

 with those of the stem. 



Cytisus Adami is probably unique, therefore, among plants in the possession of three 

 totally distinct types of leaf, each of which can readily and certainly be distinguished by the 

 naked eye, and in the possession of at least two distinct types of epidermal cell-nuclei. 



How far the hybrid part is a morphological adaptation as a mean between the two 

 parent types for physiological work it would be no easy matter to verify, but whether 

 we cau obtain indications of this in the epidermis or not, some countenance is given to 

 the view when we study sections. 



Transverse leaf sections of C. Laburnum and C. Adami are thicker than those of 

 C. purpureus in the proportion of 5 : 4. The palisade tissue of C. purpureus is quite 

 continuous over the vascular bundle of the midrib, and beneath this is a very dense 

 round-celled spongy parenchyma, so dense, however, that it scarcely merits the designa- 

 tion " spongy." In C. Adami and C. Laburnum the palisade tissue is interrupted in 

 continuity by a wedge-shaped mass of colourless cells which lie in the concavity of the 

 leaf bundle ; the spongy parenchyma is loose in the last, but in C. Adami it very closely 

 approaches C. purpureus in density. The vascular bundle of C. purpureus is small, 

 flat or slightly concave upwards, and surrounded by loose round-celled tissue the upper 

 layers of which lie below the continuous palisade parenchyma. The largest spiral 

 tracheids of the xylem measure 7 v and the average are 5 m. The vascular bundle of 

 C. Laburnum is large, semicircular, and demarcated from the surrounding laminar tissue 

 by one layer of large, clear, rounded cells. The uppermost of these, along with the 

 wedge-shaped mass of cells above mentioned, lie against the upper epidermis, and thus 

 break the continuity of the palisade parenchyma. The largest spiral tracheids are 12 m 

 across and the average are 9 /x. 



The bundle of C. Adami in shape, size, and relation to the surrounding tissue is like that 

 of C. Laburnum, but the modifying action of C. purpureus is traceable in various ways. 

 Thus the wedge-shaped cells that fill the concavity of the bundle show no thickening of 

 their walls, while the zone of rounded cells that bound the bundle is only faintly indicated. 



Sepals. — The calyx of C. purpureus is entirely green ; that of C. Adami is largely 

 green, but the tips of the sepals are semi-membranous ; that of C. Laburnum is green 

 below, but by degrees becomes membranous above, the upper third of the calyx being 

 entirely membranous. 



The inner (upper) surfaces of the sepals in C. purpureus and C. Adami resemble each 



