404 PART IV. CLASSIFICATION. 



suspensor (Fig. 239 A, 6) gives rise to the growing-point of the 

 stem, which is here lateral (Fig. 239 (7, c ; D, sf), and to that of 

 the root by a hypophysial cell (/). 



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. 238, 239) ; 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. 

 238 7?, 7?), the dermatogen-layer is completed by the periclinal 

 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. 238 D~) a cylinder of plerome 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 in tra -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 differentiation 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 stem 

 bearing the cotyledon or cotyledons, but not projecting beyond 

 them, termed the hypocotyl, passing posteriorly into (c) the primary 

 root or radicle. In some plants (e.g. Triticum and other Grasses, 

 Phaseolus, Vicia, Amygdalus, etc.) the primary stem has elongated 

 beyond the insertion of the cotyledon or cotyledons, and bears the 

 rudiments of future foliage-leaves ; this portion of the primary 

 shoot is termed the plumule or epicotyl. 



The size and texture of the cotyledons vary with the functions 

 which they have to perform. When, as in exalbuminous seeds, 

 such as peas and beans, the cotyledons are themselves the store- 

 houses in which food is deposited for the nutrition of the embryo 

 during its extra-seminal development, they are relative!} 7 large. 

 thick, and fleshy : but when, as in albuminous seeds (e.g. Ricinus, 



