SEED AND ROOT 



173 



FIG. 215. SEED OF MAIZE 

 (Zea mays). 



The testa removed to show the 

 embryo, consisting of plumule, 

 radicle, and cotyledon embedded 

 in mealy perisperm. [Note. The 

 embryo has been partly lifted 

 out from the perisperm in order 

 to show the several parts more 

 clearly.] 



The seeds of a small number of Flowering 



Plants chiefly parasites, as the Dodder (Cuscuta) 



have no cotyledons ; but these must be regarded as 



instances of vegetable degeneration, and such plants 



are classed among Phanerogams for other and 



sufficient reasons. Young plants of the Fir and 



Pine order (Coniferce) sometimes have as many as 



twelve or even fifteen seed-leaves, and thus form 



a small class by themselves, to which the name 



Poly cotyledons has been given, though the term 



would hardly be accepted by present-day syste- 



matists. Instances have been recorded of Dicoty- 

 ledons with three cotyledons (!), but such cases are 



abnormal, and should be classed among freaks of 



nature. Seedling Maples have manifested this 



peculiarity, and a speci- 

 men of a tricotyledonous 



Oak may be seen in one of the museums of 

 economic botany at Kew. 



Many curious facts have been discovered 

 by Darwin with reference to the movements 

 of plants, and not the least curious are those 

 which relate to the movements of the cotyle- 

 dons and roots of seedlings. Of the young 

 plants which he examined, the cotyledons in 

 some cases kept up a continuous movement in 

 a vertical direction ; in others they oscillated 

 from side to side, the seed-leaves always acting 

 together save, indeed, in a solitary instance, 



where one cotyledon rose while the other fell, the 



plant which exhibited this exceptional movement 



being a species of Wood-sorrel. 



The young growing rootlets likewise exhibited a 



constant slow movement from side to side,* their 



tips, which displayed the most exquisite sensitiveness, 



enabling them to avoid destruction and threatened 



injury, and to feel their way downwards between the 



particles of the soil. " A radicle," says Darwin in his 



Movements of Plants, " may be compared with a 



burrowing animal, such as a mole, which wishes to 



penetrate perpendicularly down into the ground. By 



continually moving his head from side to side, or 



circumnutating, he will feel any stone or other FlG 2 i 7 _TH E SAME IN 

 * The path is really a spiral a circumnutation. VERTICAL SECTION. 



FIG. 216. MAIZE ON THE FOURTH 

 DAY OF GERMINATION. 



The testa' has been removed. 



