SECT. II PHANEROGAMIA 515 



formation of the embryo within the ovule may commence in very various ways. In 

 most a tetrad formation acco]npanied by a reduction division takes place, but only 

 some of these ovules are found to have a normal embryo-sac capable of fertilisation ; 

 as a rule this is displaced by a vegetative cell which develops into an aposporous 

 embryo-sac. In exceptional cases apogamous embryo-sacs are formed. Thus in 

 Hieracium apogaiuy and apospory are found in addition to the normal mode of 

 development of the embryo as a result of fertilisation (cf. p. 283). It is note- 

 worthy that these plants of Hieracium in the course of generations may, after a 

 larger or smaller number of apogamous or aposporous embryos have been 

 produced, give rise to descendants which exhibit normal fertilisation. 



The Seed 



The entire structure developed from the ovule after fertilisation 

 is termed the SEED. Every seed consists of the more or less developed 

 EMBRYO developed from the fertilised ovum^ the ENDOSPERM surround- 

 ing the embryo, and the protective seed-coat. The seed-coat always 

 is derived from the integument or integuments ; their cells, by the 

 thickening, suberisation, and lignification of the walls give rise to an 

 effective organ of protection for the dormant young plant within. A 

 special development of the epidermis of the seed into mucilage cells is 

 of frequent occurrence (Quince, Limim, many Cruciferae, species of 

 Plantago, Cuphea, Cobaea, etc.). The mucilage serves as a first means of 

 fixation in the soil and also retains water which is necessary for ger- 

 mination. Such other features of the surface as hairs, prickles, etc., 

 have usually the former function, if they do not stand in relation to 

 the distribution of the seed. 



Points of morphological importance in the seed-coat have been 

 referred to in considering the ovule; micropyle, hilum ( = place of 

 attachment to the funicle), and raphe. From what was said above 

 (p. 476) it follows that the micropyle and hilum will lie at opposite 

 poles of the seed when the ovule is atropous (Fig. 473). In seeds 

 derived from anatropous ovules (i.e. those in which the funicle lies 

 along one side of the ovule, which is bent round at the chalaza) the 

 hilum and micropyle are close together (Fig. 474). Only seeds of this 

 kind possess a raphe connecting the hilum and chalazal region. 

 Campylotropous ovules develop into seeds resembling those derived 

 from anatropous ovules. 



In some cases the function of the seed-coat is modified owing to the protection of 

 the seed or seeds being undertaken by the pericarp ; the innermost laj^ers of the 

 latter are developed as sclerotic cells and form the stone of the drupe (cf. p. 520). 

 In such cases (e.g. Almond, Cherry-Laurel, Cerasus, Piper, etc.), since any special 

 development of the seed-coat is unnecessary, it tends to become reduced ; its cells 

 do not thicken or modify their walls and the various layers become simply 

 compressed. 



The nutritive tissue in the seeds is developed, as a rule in the case 

 of Gymnosperms, by the time of fertilisation and constitutes the 



