MINUTE STRUCTURE OF PLANT HYBRIDS. 



215 



some hybrids over either parent has arrested attention, and it has been accounted for by 

 supposing that the strength which is not spent in fruit production passes into the vegetative 

 parts. For many reasons this can scarcely be accepted as a true explanation, and the case 

 now adduced leads us rather to believe that its cause is to be sought for in some deep-seated 

 cell condition which exists before the reproductive organs have appeared. Evidence 

 could easily be adduced to prove that it as often exists in fertile as in sterile hybrids ; 

 probably even more so in the former, if my data give a true index to the entire range 

 of hybrids, while Focke shares the same opinion.* I would suggest that an explanation 

 is to be had rather from the standpoint of exaggerated cross-fertilisation effects, and 

 that increase of the hybrid over the parents is due to increase in size of cells rather 

 than to increased multiplication of them. 



The lower epidermis (Plate III. fig. 8) is very neatly intermediate in shape of the 

 cells and disposition of stomata. The former have a waviness derived from 1, but also 

 a transverse extension usually which is derived from 3. The stomata, though 

 partaking somewhat of the irregularity of 1, can readily be followed in undulating 

 lines as in 3. But under the same area as above there are only eighteen to twenty 

 stomata, which would be explained if we take into account the increased size of the 

 epidermal cells. At the same time such a distribution suggests points of great com- 

 plexity relating to transpiration, &c, which we cannot here enter into. 



I would draw attention, however, to the cuticular warting, which is, as nearly as one 

 can measure, half as strongly developed as in Philesia, and, like that parent, is most 

 strongly formed on the slightly arching sides of the epidermal cells. 



Several authors assert that there are fundamental differences in the leaf venation of the 

 parents. Eeference to Plate III. figs. 3, 2, and 1, will show that both are modifications 

 of a common fundamental type, and that the hybrid exactly connects them in distribution 

 of the large and small veins alike. The leaf venation, in truth, coincides with other parts 

 in teaching us that Lapageria is a form specialised for living under mild environmental 

 conditions, while Philesia is equally specialised for rigorous atmospheric surroundings. 

 The reduction in the number of longitudinal vascular bundles in Philesia, their greater 

 size and strength, their firm union at the apex, the strong nature of the transverse or 

 oblique girders, and their reduction in number, all point to derivation from a type 

 nearly like the hybrid, and that alteration has occurred to suit the 'peculiar climatic 

 surroundings which Darwin, Agassiz, and others have depicted as existing on the 

 "West Fuegian coast. 



Transverse sections of 1, taken at equal distances from apex and base of the leaf, 

 show that the epidermal cells just over the median vascular bundle, though narrower than 

 the average of those over the leaf surface, are quite as deep. They are separated from the 

 indurated sheath of the bundle by one or two layers of rounded cells continuous with those 

 of the palisade parenchyma. In 3 the epidermis over the median bundle is depressed, and 

 its cells are greatly reduced in width and depth, so that they form a median line very 



* Pflanzen-mischlinge, p. 475. 



1. Lapageria 



rosea. 



2. Philageria 



Veitchii. 



3. Philesia buxi- 



folia. 



