96 THE ORIGIN OF MONOCOTYLEDONS [CHAP. 



With regard to the development of the 

 embryo, the first noticeable part is the arrest 

 of the primary or axial root, with the subsequent 

 production of adventitious roots in an ascend- 

 ing series from the stem, a universal feature 

 in monocotyledons. The stem thus assumes 

 the form of a cone tapering downwards. It 

 is characteristic of aquatic dicotyledons, as 

 in Nymphceacece, Umbelliferce, Ceratophyllum, 

 Ranunculus heterophyllus, etc. Mirbel, 1 in 1810, 

 noticed that the arrest of the root was correlated 

 to that of the cotyledon as having lost its sources 

 of nourishment ; but we may go further. Since 

 the development of roots depends upon the 

 foliage, it is easy to see that when cotyledons 

 are raised above ground, and act as leaves, as 

 in mustard and cress, they can maintain the 

 existence of the tap-root and enable it to grow 

 until the foliage of the plumule is developed. 

 In submerged aquatic plants, however, the first 

 leaves are more or less rudimentary and arrested 

 in character. Moreover, being under water and 

 the protoplasm weakened by supersaturation, 

 they are unable to develop organs equal in degree 

 to those of aerial plants. The consequence is 

 that the primary root, which must be supplied 

 with nourishment from the first formed leaves, 

 suffers and perishes. This is true for aquatics 

 of both classes ; but having become a fixed and 



1 " Examen de la Division des Vegetaux en Endorhizes et 

 Exorhizes," Ann. Mus. d'Hist. Nat., 1810. 



