MINUTE STRUCTURE OF PLANT HYBRIDS. 



229 



is fully able to dispute with both parents for occupation of territory. If species evolution 

 from forms of hybrid origin does occur, this is one of the most likely to illustrate it. 



(d) Ribes Culverwellii, x . 



This remarkable cross of two very distinct species, the gooseberry and black currant, 

 was effected by Mr Culverwell of Thorpe Perrow, Yorkshire. He forwarded specimens 

 to Dr Masters, who duly described them in the Gardeners' Chronicle* and then kindly 

 forwarded me the material. This proved so instructive that I applied to Mr Culverwell 

 for further supplies, and he has furnished these at different seasons of the year. He 

 distinguishes two or three varieties, which differ from each other in leaf form, time of 

 defoliation, and habit. I selected for examination that which appeared to be most nearly 

 intermediate between the parents, but the others which inclined to the black currant 

 parent will be treated of at another time. 



Stem. — On transverse section of a stem of 1 in Spring of the second year's growth 

 (Plate V. fig. 7), the epidermal cells are rather broader than deep, of a whitish colour, 

 and covered by a thick cuticle. 



The outer part of the cortex consists of cells which are small, densely packed, and 

 colloid just beneath the epidermis ; but by degrees they become larger, looser, and assume a 

 brown colour internally till in the seventh to ninth layers they break down into large 

 irregular reticulations loosely attached to the outer cork. The cork, already showing a 

 second year's growth, varies greatly in amount according to the size and vigour of the 

 shoot. The specimen figured was from a small shoot, and had produced four to five zones 

 of cells each year. This explains the apparent anomaly of the hybrid in having eight to 

 ten corresponding zones. The old cork cells are 3 to 3^ times as broad as deep. The 

 inner cortex (phelloderm) is a relatively narrow band of tissue seven to eight zones 

 deep, and made up of elements differing greatly in size. 



Sections of 3 (fig. 9) show epidermal cells that are deeper than wide, of a brown 

 colour, and which have a thin cuticular layer. 



The outer part of the cortex consists of a mass of loose, shrivelled-looking brown cells 

 that are considerably flattened. The cork of each year's growth is made up of cells that 

 are 1^ to 2 times as broad as deep. The inner cortex is a deep band of tissue made up of 

 fifteen to sixteen cell layers. 



Sections of 2 (fig. 9) show an intermediate state, not only in the size and shape of the 

 cells, but also in colour distribution to the epidermis and inner cortex region. 



The phloem in 1 is in depth as 7 to 10 in 2 and 13 to 14 in 3. The sieve tubes in 1 are 

 rather sparse, 1 2 ft across, and are embedded among companion cells with thick refractive 

 walls. In 3 the sieve tubes are abundant, 20 to 21 fx across, and are surrounded by a 

 moderate number of companion cells with thin walls. In £ the sieve tubes are pretty 

 abundant, and the companion cells have slight thickening in their walls. Though the 



* Gardeners' Chronicle, vol. xix., n.s. 1883, p. 635. 



1. Ribes Grossu- 

 laria. 



g. Ribes Culver- 

 wellii. 



3. Ribes nigrum. 



