400 



FOURTH GROUP. SEED-PLANTS. 



W 



former the cotyledon is terminal, while in the latter the two cotyledons are of lateral 

 origin at the upper extremity of the embryo, though they often, as for instance in 

 Capsella, take up so much of the upper part of the embryo that the plumule cannot 

 be recognised as a distinct projection between them. Moreover there are monocoty- 



ledonous embryos, as Solms- 

 Laubach has shown 1 , in which 

 the cotyledons are not a terminal 

 but a lateral formation on the 

 embryo. This is the case in the 

 Dioscoreaceae and some, perhaps 

 all, the Commelinaceae. In these 

 plants the vegetative cone of the 

 stem originally occupies the ex- 

 tremity of the embryo, and is 

 afterwards moved into a lateral 

 position by the development of 



FIG. 331. Embryos of Allium Cepa in different stages of development. In 



/ the spherical cell at the end of the suspensor contains two nuclei. In // it has the COtyledon which IS formed 

 divided into a' and a", and t in / into c and <? in //; x is the remains of one of 

 the synergidae, embryo-sac.wall. bdlCath, that is, at the side Of the 



vegetative cone of the stem 2 . 



Again, some embryos are not borne on a suspensor. The oospore of Pistia 

 for example is transformed into a spherical cellular body, which directly represents 

 the embryo. On the other hand, in some Orchideae the suspensor assumes 

 the peculiar form of a row of cells with transverse walls which grows out of the 

 micropyle and attaches itself to parts like the placentas, where food-material is 

 present, in order to convey it to the embryo 3 . In species of Lupinus the cells of the 

 long suspensor separate at an early period of their existence, and the embryo then 

 lies free in the embryo-sac at a distance from the micropyle 4 . 



There are many variations also in the external form of the embryo and con- 

 sequently in the arrangement of its cells, and also in the mode of formation of the 

 root-cap. These variations are found, as we learn from Hegelmaier 5 , in allied plants, 

 and are connected with the number of the cells of the pro-embryo which are employed 

 in the formation of the embryo ; in the Cruciferae and other plants the number of 

 cells is two (compare Capsella], in other cases three or more. The differentiation of 

 the apex of the root takes place in some cases, as in the Gramineae, deep within the 

 tissue of the embryo, and the apex is therefore covered by a layer of tissue which it 



1 H. Graf zu Solms-Laubach, Ueber monocotyle Embryonen mit scheitelbiirtigem Vegetations- 

 punkt (Bot. Ztg. 1878, p. 65). 



2 As far as the morphological nature of the cotyledons is concerned it is a matter of indifference 

 where and how they are formed in the embryo, for that they must be considered foliar structures is 

 established by the fact that they are often but slightly different in the fully developed state from the 

 first foliage-leaves. 



3 Treub, Notes sur 1'embryogenie de quelques Orchidees (Naturkund. verhandl. d. k. Akad. 

 Deel XXI, 1879). 



4 Strasburger, Ueber vielkernige Zellen u. Embryogenie von Lupinus (Bot. Ztg. 1880). He- 

 gelmaier, in same place. 



5 Hegelmaier, Vergl. Unters. ii. Entw. dicotyl. Keime, Stuttgart, 1878. 



