*34 



THE FLOWERING PLANT. 



which is a brown layer covering the edible endosperm, here form- 

 ing a relatively thin layer surrounding a milk- containing cavity. 



At one end of the cocoa-nut 

 are three round marks. One 

 of these corresponds to a soft 

 part adjoining the micropyle 

 of the seed. The minute em- 

 bryo is to be found imbedded 

 in the endosperm at this point. 

 The various parts of the seed 

 are subject to considerable 

 variation, as the above ex- 

 amples show. Straight, bent, 

 and inverted ovules develop 

 into similarly termed seeds 

 (cf. p. 104). The seed-coat is 

 of one or two layers, according 

 to the number of integuments 

 possessed by the ovule. It 

 frequently happens that out- 

 growths from the seed-coat or 

 funicle are present. These 

 constitute an aril, which may 

 consist of hairs only, as in 

 cotton, willow-herb, and wil- 

 low ; or it may be a knob 

 (castor-oil seed), ridge, crest, 

 or complete extra covering. 

 Examples of the last kind are 

 seen in the red fleshy cup en- 

 closing the solitary seed of 

 yew, the loose orange-coloured investment of spindle-tree seeds, 

 and " mace " which surrounds the nutmeg. 



Both in albuminous and exalbuminous seeds, but especially in 

 the latter, the embryo may be packed away in the most various 

 manners. The cotyledons particularly are often folded, rolled, or 

 crumpled in an elaborate way. 



Little need be said here about the minute structure of the 

 seed. It is enough to state that reserve materials are laid up in 

 the albumen (or cotyledons if this is absent) under three chief 

 forms: (1) starch grains; (2) aleurone grains, which are minute 

 masses of proteid matter, often containing crystalloids (cf. p. 26) ; 

 (3) °My matters; (4) cellulose. The date is an example of (4); 

 and oily seeds, such as brazil-nut and castor-oil, of (3). Aleurone 

 grains are commonest and largest in oily seeds. A thin section, 



Fig. 54.— Longitudinal Section 

 (grain) of Maize {Zea Mais), x 4 [from Sachs], 

 c. pericarp ; n. remains of style ; fs. attached 

 base of fruit; eg. dense yellow endosperm, 

 between which and the pericarp is seen the 

 seed-coat; eiv. loose white endosperm; sc. 

 scutellum of embryo, its continuation can 

 be traced round the embryo below; g«. its 

 apex ; e. its epidermis (shaded like seed-coat) 

 on endosperm side; k. plumule ; w. (below) 

 radicle, tipped by r<>ot-cap; ws. its root- 

 sheath ; to. (above) adventitious roots arising 

 from embryonic stem. Vascular bundles, 

 white (note pith in vascular cylinder of radi- 

 cle, as well as in stem). 



