192 



MORPHOLOGY OP ANGIOSPERMS 



it we would cite Sparganium (Campbell 55 ), Potamogeton 

 (Wiegand, 63 Ilolferty 71 ), Zannichellla and Naias (Camp- 

 bell 41 ), Triglochin (Hill 60 ), and Limnocharis (Hall 82 ). The 

 last-mentioned form well illustrates that the general type may 

 be maintained, and at the same time there may be no regularity 

 in the sequence of divisions after the first two. In fact, the 

 apical cell of the proernbryo of Limnocharis may divide by a 

 transverse, oblique, or longitudinal wall, and in the two latter 

 cases the cotyledon and stem-tip are both terminal, as is the 

 case also in Zannichellia. 



Among the Gramineae the same general type of proernbryo 

 is formed, but if Arena fatua (Cannon 65 ) be taken as repre- 

 sentative of the general situation, the origin of the organs of 

 the embryo in relation to the cells of the proernbryo is quite 

 different. In this species the cotyledon and stem-tip are both 

 derived from the apical cell, the entire root-tip (including root- 

 cap) from the adjacent cell, and the coleorhiza from the third 

 cell, the suspensor consisting of only the primary basal cell. 



Among the Araceae a very different type of embryo is indi- 

 cated, but so few forms have been investigated that no conclu- 

 sion as to its prevalence in the family is safe. In 1874 Hegel- 



maier 9 described the absence of 

 a suspensor in Pistia, the fertil- 

 ized egg producing a spherical 

 proernbryo, all of which enters 

 into the structure of the embryo. 

 Campbell 59 found the same type 

 of embryo in Dieffenbachia, Ag- 

 laoncma, and Lysichiton (Fig. 

 84), and states that in the seg- 

 mentation of the egg there may 

 be two transverse divisions be- 

 fore any vertical division, or a 

 regular quadrant may be formed 

 as in the ordinary fern embryo. 

 Even if this Pistia type should 

 prove to be characteristic of the 



Fig. 84. — Lysichiton hamtschatcense. 

 Longitudinal section of embryo sur- 

 rounded by endosperm, illustrating 

 the Pistia type.— After Campbell.™ 



Arac 



eae, it is not restricted to them, for Humphrey 



has 



shown that the embryos of the Scitamineae have no suspensors ; 

 and the same is true at least of certain orchids, as shown by 



