CHAP. L 



ALLIUM. 



59 



by us of the part that first breaks through the ground not 

 being arched. 



Fig 46. 



Canna Warscevciczii : circumnutation of plumule with filament affixed 

 obliquely to outer sheath-like leaf, traced in darkness onhorizontal glass 

 from 8.45 A.M. Nov. 9th to 8.10 A.M. llth. Movement of bead mag- 

 nified 6 times. 



Allium cepa (Liliacese). The narrow green leaf, which pro- 

 trudes from the seed of the common onion as a cotyledon,* 

 breaks through the ground in the form of an arch, in the same 

 manner as the hypocotyl or epicotyl of a dicotyledonous plant. 

 Long after the arch has risen above the surface the apex 

 remains within the seed-coats, evidently absorbing the still 

 abundant contents. The summit or crown of the arch, "when 

 it first protrudes from the seed and is still buried beneath the 

 ground, is simply rounded; but before it reaches the surface 

 it is developed into a conical protuberance of a white colour 

 (owing to the absence of chlorophyll), whilst the adjoining parts 

 are green), with the epidermis apparently rather thicker and 

 tougher than elsewhere. We may therefore conclude that this 

 conical protuberance is a special adaptation for breaking through 

 the ground,f and answers the same end as the knife-like white 

 crest on the summit of the straight cotyledon of the Graminese. 



* This is the expression used 

 by Sachs in his 'Text-book of 

 Botany.' 



t Haberlandt has briefly de- 

 scribed (' Die Schutzcinrichtun- 

 gen . . . Keimpflanze,' 1877, p. 77) 

 this curious structure and the 



purpose which it subserves. He 

 states that good figures of the 

 cotyledon of the onion have been 

 given by Tittmann and by Sachs 

 in his 'Experimental Physiologie,' 

 p. 93. 



