MINUTE STRUCTURE OF PLANT HYBRIDS. 



243 



stems, slightly, if at all, thicker than the leaf base, which rise above the root tufts. Each 

 is really a small pseudo-bulb, as has been correctly represented in the drawing of the 

 Botanical Magazine cited above. Miss Woolward is therefore wrong in attempting to 

 criticise the drawing adversely and to deny the existence of such a growth.* Both 

 parents exhibit, in the size and colouring of the flower parts, considerable variability, as 

 was demonstrated by specimens kindly sent me from Glasnevin Gardens, from Mr 

 Veitch of Chelsea,, and Mr Buchanan of Oswald Koad. It is not surprising, therefore, 

 that the published figures and descriptions do not all agree with each other. The 

 microscopic details which give rise to these macroscopic differences are chiefly variations 

 in the number and distribution of coloured hair-cells. 



Root. — On transverse section an external velamen of four layers shows cells in 1 greatly 

 widened out tangentially, each being three and a half to four times broader than deep. 

 In 3 the cells are one and a half to two times broader, and in 2 they are two and a half to 

 three times broader. The cells of the epidermis in 1 have their external and lateral walls 

 strongly cuticularised in U-shaped manner, those of 3 are only slightly cuticularised exter- 

 nally, while 2 shows an intermediate amount both of lateral and external cuticularisation. 



In 1 the root cortex cells have clear colloid walls, specially thickened at the angles, 

 as in typical collenchyma, and small intercellular spaces are enclosed by the thickened 

 portions. In 3 the cortex cells are large and loose, with rather large intercellular spaces 

 enclosed ; the walls throughout are thin, and even at the angles there is little colloid 

 thickening. The cells of 2 are intermediate in size, amount of colloid thickening, and 

 development of intercellular spaces. 



In 1 the cells of the bundle sheath and of the xylem are strongly thickened, so that 

 the lumen — specially of the xylem elements — is greatly reduced. In 3 these elements 

 are of large size and are slightly thickened, while in 2 all the roots that I have yet 

 examined approach nearer to 3 than to 1. In 1 the pitted vasa are 18 /a across, in 3 

 they are 35 /a, and in 2 they are 28 /u. 



Longitudinal root sections of 1 show cortex cells from the third layer inwards, which 

 are greatly elongated and have thick, white, colloid walls. In 3 these are loose, thin- 

 walled, slightly irregular in shape, and are, on the average, rather broader than long. 

 In 2 the cells are considerably longer than broad, and show a moderate degree of 

 colloid thickening. 



Stem. — Only a few details need be referred to here. On transverse and longitudinal 

 section of the cortex of 1, the cells are greatly elongated ; their walls are thickened so 

 that slit-like deficiencies with pore apertures are left, and between the cells are small 

 intercellular spaces. In 3 the cells are slightly longer than broad, many have no 

 thickening deposits, but a few show a very pretty secondary spiral thickening. Many 

 of the cells surround large intercellular spaces. In 2 the shape and size of cells, as also 

 the size of the intercellular spaces, are not only intermediate, but spiral cells like those 

 of 3, though with less thickening deposit, are distributed among the unthickened ones. 



* " The genus Masdevallia." 

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



1. Masdevallia 



amabilis. 



2. Masdevallia 



Chelsoni. 



3. Masdevallia 



Veitchiana. 



