— 351 — 



Farcraea gU/nnfea Watsoniaita. D r u m m o n d, Rept. Mo. Bot. Gard. IS : 59. 

 (1907). 



Variegation creaniy-wliite, median, internipted by stripes of green. 



Apparently the Kew specimen is now unique. 



Structure. 



The variegations that have been observed in Agaveae are 

 primarily etiolations of the normally green subepidermal parenchyma, 

 producing a greenish white or milk white effect according to its 

 completeness or the depth of its penetration below the epidermis 

 which itself is deprived of chloroplasts except for the sunken 

 guard cells of the stomata. Yellow or juvenile rosy recolorations 

 of the etiolated parts result respectively from the rather sparing 

 development of a plastid or cell-sap pigment. 



A section across a thick leaf of the americana type shows to 

 the naked eye that its normal coloration is mainly confined to a 

 deep-green zone scarcely exceeding 1 — 1-5 mm in thickness on the 

 Upper surface, and noticeably thinner on the lower surface when this 

 is turned from the light. Though a tissue of markedly elongated pali- 

 sade cells is absent, the nearly isodiametric cells of this green zone are 

 arranged in regulär series perpendicular to the epidermis. Under this, the 

 shade of green is much paler,either throughout the blade when this is not 

 very thick or fading away toward its center in the thicker or basal part, 

 with a persisting tinge immediately about some of the fibro-vascular 

 bundles. The darker coloration appears to mark the effective depth of 

 penetration of actinic light through the chlorophyll-charged tissue; and 

 the disappearance of all coloration, the ultimate limit of its pene- 

 tration. In variegated leaves, on the other hand, it is noticeable 

 that fully colored green bands may immediately underlie the etio- 

 lation or even be found at all depths in the mesophyll under the 

 translucent Windows afforded by large median blanched stripes. 



The direct cause of the marked differences in the green 

 coloration lies in the plastid contents of the cells. In the outer 

 deep-green zone the chloroplasts are numerous and commonly 

 large, often measuring 7 — 10 \i in diameter: below it they become ab- 

 solutely and relatively less numerous and less pigmented, and they 

 are smaller, scarcely reaching 4 y^ in diameter, — a condition that 

 has been attributed by Zimmermann to variegation-bianching 

 in oeneral. 



