DTV. H 



SPERMATOPHYTA 



579 



The Seed 



The entire structure developed from the ovule after fertilisation 

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

 EMBRYO developed from the fertilised ovum, the ENDOSPERM surround- 

 ing the embryo, and the pro- 

 tective SEED-COAT. The seed- 

 coat always is derived from 

 the integument or integu- 

 ments ; their cells, by the 

 thickening, suberisation, and 

 lignifi cation of the walls, give 

 rise to an effective organ of 

 protection against drying and 

 injury for the dormant young 

 plant within. A special de- 

 velopment of the epidermis of 

 the seed into mucilage cells 

 is of frequent occurrence 

 (Quince, Linum, many Cruci- 

 ferae, etc.). The mucilage 

 serves as a first means of 

 fixation in the soil and also 

 retains w T ater which is neces- 

 sary for germination. 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 

 are (1) the MICROPYLE, (2) 

 the HILUM ( = place of attach- 

 ment to the funicle), and (3) 

 the RAPHE. From what was 



Said above (p. 540) it follows FlG> 5(32 -~ Aposporous origin of the embryo -sac of 



that the micropyle and hilum 



will lie at opposite poles of 



the seed when the ovule is 



atropous (Fig. 508). 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. Only seeds of this kind 



possess a raphe connecting the hilum and chalazal region. Campylo- 



Hieracium flagellare. o, Normal tetrad of macro- 

 spores ; 6, c, the disorganisation of this. The 

 diploid embryo-sac arises from a cell of the integu- 

 ment that is recognisable in a. (After ROSENBERG 

 and A. ERNST, 1918.) 



