GROUP IV. — PHANEROGAMIA. 445 



embryo. In Monocotyledons, on the other hand, the embryo-cell 

 gives rise, as a rule (Fig. 287 A and C, a), only to the single ter- 

 minal cotyledon ; whilst the last cell of the suspensor (Fig. 287 

 A, b) gives rise to the growing-point of the stem, which is here 

 lateral (Fig. 287 0, c ; D, st), and to that of the root by a hypo- 

 physial cell (/). 



In two cases only {Ccphalotaxus Fortunei, Araucaria brasiliana, both Gymno- 

 sperms) are the cotyledons and the growing-point of the primary stem 

 developed endogenously : here they are at first covered by some cells at the 

 apex of the embryo, which are eventually thrown oflf. 



In a few exceptional Monocotyledons {e.g. Dioscoreaceee, Coramelynaceag) the 

 growing-point of the primary stem is developed, not laterally, but apically, 

 and the cotyledon is lateral. In some Dicotyledons {e.g. Canim Bulbocastanum, 

 }ia7iuncuhis Ficaria) the embryo is pseudo-vionocotyledonous\ that is, only one 

 cotyledon is developed though two are originally indicated. 



In the Gymnosperms, the number of cotyledons varies from one of fifteen. 



With regard to the histological differentiation of the embryo, 

 the first step, after the division into octants, is the formation of 

 periclinal walls marking off a superficial layer, which is the 

 dermatogen (Figs. 286, 287) ; this differentiation proceeds from 

 the anterior end, or apex, backwards towards the posterior end of 

 the embryo. In those plants in which the root-end of the embryo 

 is formed by a hypophysial cell contributed by the suspensor 

 (Fig. 286 B, h), the dermatogen-layer is completed by the peri- 

 clinal division of the hypophysial cell, the inner cell forming the 

 periblem of the growing-point, the outer forming the dermatogen 

 which undergoes further periclinal division to form the primitive 

 root-cap. In the meantime, anticlinal and longitudinal walls 

 have also been formed, so that the embryo, as it increases in size, 

 consists of an increasing number of cells. The degree of histo- 

 logical differentiation attained varies widely : in the highest forms 

 (Fig. 286 D) a cylinder of plerorae is differentiated in the axis 

 of the embryo, so that the three primary tissue-systems, der- 

 matogen, periblem, and plerome, are clearly defined. 



The degree of morphological differentiation attained by the 

 embryo in its intr-a-seminal development also varies widely, as 

 does also the size of the embryo. In the ripe seed of most Orchids 

 and parasitic plants {e.g. Orobanche, Monotropa, etc.), the body of 

 the embryo presents no differehtiation into members. In most 

 plants, the embryo, in the ripe seed, consists of the following 

 members : (a) one, two, or several cotyledons ; (6) a primary 



