MINUTE STRUCTURE OF PLANT HYBRIDS. 



223 



out all the cells of the cortex. In 3 the cells are nearly all shorter than deep, their 

 length is 15 to 25 /x, and they are irregularly rounded in outline ; between them many 

 irregular intercellular spaces occur. The chloroplasts measure 5 /x, and are densely 

 aggregated in the outer zone, giving to it a dark green aspect. In 2 the cells are distinctly 

 elongated, and measure 40 to 45 ti in length ; irregular intercellular spaces are pretty fre- 

 quent, and a marked massing of chloroplasts takes place in the outer zone, while each 

 chloroplast measures about 4 /x. The parents therefore differ, not only in the size, shape, 

 and relation of these cells, but in the amount of elaborative work which they perform, 

 while the hybrid progeny blends these peculiarities. 



A reverse case is presented by the inner cortex, for in / the elements are one-third 

 the size of those in 3. 



The elements of phloem and xylem differ greatly in size, the small elements of 1 

 contrasting with the large elements of 3, while the hybrid is a mean of the two. But 

 even the amount of secondary deposit on the spiral tracheal wall can be readily traced 

 to stand in the relation of 7 : 8 : 9 in the three. 



Leaf. — The leaf structure of 3 varies considerably at different levels in the same strain 

 or variety, and very greatly in the numerous varieties now cultivated in our gardens. 

 The description which follows applies to the variety already selected. I regard the upper 

 two-thirds of the leaf as the more typical part, and shall refer to it throughout. The 

 average depth of the leaf in the three on transverse section through the midrib is as 

 2:8:15; the thickness a little to one side of the midrib is as 360 /x : 400 /x : 440 tt. 



On transverse section the upper epidermal cells in 1 are 35 to 40 xi in depth ; in 2 they 

 are 55 to 60 /x ; in 3 they are 70 to 80 /x. The palisade parenchyma in 1 consists of three 

 layers of round, columnar cells with small vertical intercellular spaces, and the loose 

 parenchyma is divisible into a lower dense and an upper loose zone, the chloroplasts in the 

 lower being nearly as abundant as in the palisade layer. But the loose parenchyma 

 throughout consists of isodiametric cells with small intercellular spaces. In 3 the palisade 

 parenchyma forms two rather loosely united layers, which thin out at times into one, or 

 enlarge into three ; the loose parenchyma is uniform throughout, and its cells are elongated 

 at right angles to the leaf surface and strung together in " confervoid " fashion with large 

 intercellular spaces between. In 2 the palisade parenchyma shows two to three rather 

 loosely aggregated cell layers ; the loose parenchyma is distinctly divisible into a lower and 

 upper area, the former with greater abundance of chloroplasts, but the cells, though often 

 nearly isodiametric, form "confervoid" strings. 



In 1, four to six large conglomerate crystals may be exposed in section; in 2 from 

 twenty to twenty-five ; and in 3 from seventy to eighty. 



In Plate IV. figs. 1-6, the surface appearances of the upper and lower epidermis are 

 given. These speak for themselves, but the stomatic distribution may be summed up in 

 figures. Within a limited area, repeatedly verified from different leaves, seventy-three 

 stomata occurred in 1, thirty-eight in 2, and two occurred in 3. The number is reversed, 

 however, over the lower epidermis, which gave under a high power at one-quarter from 



1. Dianthus 

 alpinus. 



3. Dianthus 

 Grievei. 



3. Dianthus 

 harbatus. 



