MINUTE STRUCTURE OF PLANT HYBRIDS. 



211 



isodiametric cells 18 to 20 xi across; their walls are thickened by five to six lamellae, 

 which enclose a circular or oval lumen (fig. 10c). In 3 (fig. 7) the cells are radially 

 elongated, measuring 48 to 50 /x in radial direction, and 35 to 40 /x tangentially, so that 

 each cell is barely one and a half times deeper than wide ; there are eleven to twelve 

 highly refractive lamella?, while the almost obliterated lumen of the cell is an elongated 

 slit (fig. 10a). In 2 (fig. 8) the cells are mostly elongated radially, though rarely one 

 finds a nearly isodiametric cell ; they measure 35 /x in depth and 20 to 22 /x in width. 

 Each has eight or nine lamellae, which are less refractive than in 3 (fig. 106). 



The pericambium in all is very much alike, and its constituents soon undergo 

 thickening and conversion into permanent tissue. 



The phloem patches of 1 (fig. §ph) vary from twenty-three to twenty-eight in all 

 mature roots examined ; those of 3 are eight to ten in number ; while in the hybrid there 

 may be seventeen to twenty patches. 



The xylem of 1 has its wood tracheids most strongly thickened in a circular area 

 which extends round internal to the phloem, while the radiating xylem spokes show pretty 

 large spiral tracheids. The centre consists of cells that are slightly thickened, and which 

 form a root pith. The pitted vasa are large, numerous, often disposed in groups of 

 two to three, and average 60 /x across. The xylem of 3 shows thickest tracheids in the 

 middle, with narrow, irregular radiating spokes passing out between the phloem patches. 

 One pitted vas, or rarely two, occurs in the angle between each pair of phloem patches, 

 and measures 37 to 40 /x across. The xylem of 2 is made up of tracheids pretty uniformly 

 thickened, or rather less so internally than externally ; the pitted vasa are more numerous 

 than the phloem tracts, and are occasionally grouped in pairs, very rarely in triplets. 

 Each measures 48 to 50 /x. 



The root, therefore, is very exactly intermediate in the tissues outside the bundle cylinder, 

 but the cylinder itself slightly diverges towards the seed parent in some of its features. 



[I do not refer in detail here to longitudinal views of the root elements, as they 

 fundamentally resemble those of the stem, which will now be examined.] 



Stem. — The descriptions of this have been taken from preparations made at a level of 

 one inch above ground. As already stated briefly, the glaucous stem of 1 is roughened 

 both by longitudinal ridges and by a close-set series of wart-like papillae, that are often 

 cut through in transverse sections. The stem of 3 is quite smooth, except that here and 

 there minute warts similar to those of 1, but greatly reduced in size, are sparingly pre- 

 sent. In the hybrid, as we shall see, there is a very exact blending of the two conditions. 



Transverse and longitudinal sections of 1 show epidermal cells whose free surfaces are 

 traversed by four to six of the ridges above mentioned, each measuring 5 /x in depth 

 (Plate II. figs. 3,6). The cells have grown out in a part of the figure cited, so as to form 

 one of the papillae which are distinguished with the naked eye. Each cell is isodia- 

 metric or slightly columnar, and while the outer surface is strongly cuticularised the lateral 

 faces are little altered. The average size of each is 60 /x long by 35 /x wide and 40 /x 

 deep. In 3 the free faces of the cells are quite smooth (figs. 1, 4), and the thick cuticle is 



VOL. XXXVII. PART I. (NO. 14). 2 I 



Lapageria 



rosea. 

 Philageria 



Veitchii. 

 Philesia buxi- 



folia. 



