on of Angiosperms ; The Seed Stage. V. 



The Seed, as the essential factor in the life-history of the dominant Und^lant 

 (Phanerogam), represents an ingenious solution of the problem of starting a new 

 individual of the same class of organism as the parent plant rooted in the sod, so that 

 the sexual stages as inherited from an older aquatic phase of 



fVMVt ff^^tJtlw AflfcJfel^^B^a^fl * 



exposed t o the atmosphere, but remain completely enclosed in the 

 protected within successive investments : le. : (i) The ovary, as the 

 of Angiosperms, encloses the ovules (megasporangia) : (s) the latter retain their sole 

 rsittiaJ uiegaspore (' embryo-sac 1 ); (3) the embryo-sac encloses the fe 

 s vestigial construction : (4) of which one nucleus represents the 



In the oldest environment of the sea the oosphere was fredy discharged in the 

 to be fertilised by a flagellated spermatosoid, as are the ova of fishes to the pec 



To secure dispersal in the absence of any mechanism for the 

 embryo, the latter, enclosed in vestigia of embryo-sac, and ovular struct 

 at the hilum, as the general case: provided with pro. vestment (testa) 



its perennaiing condition, and supplied with food-malerial for use on 

 until it establishes a full autotrophic mechanism of leaves and root. 



Storage of Pood Reeenree i Food-supply, once set going by the V.B. of the 

 ovule, may be in excess of the amount required for immediate growth of the embryo, 

 and hence accumulates to be utilised at a later date. The manner of 4fpotfrfoft 

 largely depends on the relative rates of digestion in endosperm drawing on niMilliM. 

 and embryo drawing on endosperm. If the endosperm is more active, it may fiO up 



(case of the e 



with supplies, and the embryo remains small (case of the endospermk or 

 seed). If the embryo digests supplies actively the endosperm may be poorly developed, 

 all used up, or ultimately completely wanting in the seed-stage (case of ' ^^rhnmtf^^' 

 or non-endospcrmic seed). All grades occur: a state in which the endosperm is 

 reduced to i-a layers only, and hence inconspicuous except in section, b very general 

 Endosperm is wholly wanting in seeds of most Leguminosae, and in Compositae. As 

 general cases storage may be conspicuous: 



( i ) In Endosperm ; Grasses, Coco-nut, Rtcimu (the oldest condition). 

 i In Embryo, more particularly in massive cotyledons (Bean, Sunflower). 



(3) In parenchyma of the nuccllus, as ' Perispcrm ', cf. Water Lily, Centrosperms. 

 Nature of Respires ; Mainly excess carbohydrate material, with smaller 

 amount of proteid (nitrogenous reserves). Soluble materials are excluded (hence no 

 nitrates, no potassium compounds, and no sugars) : general cases include : 



1 i ) Carbohydrates as starch (Bean, Wheat, and Cereals) ; CV/rWw, as almost solid 

 wall (Date) ; Reserve-celluloses of many kinds (Aac*^ PkyttUpkat\ 



(2) Hydrocarbon fats, as compounds of fatty acids and glycerine : cf. Sunflower. 

 Poppy, Rapeseed, Castor Oil, Palm Oil : the commonest case. 



(3) Proteid, localized in general cytoplasm of the storage cells, or in special granules 

 (Alfuront grains) ; these may be differentiated in the dry seed, with crysuBoid 

 of globulin in ground substance (Rtfinus, Braxil-nut). 



(4) Mineral Matter may be associated with such aleurone grains hi special form of 

 sphaerocrystals, as the ' globoid ' of CaMg Phosphate. 



Porennation of the Seed: The embryo passes into a resting stage ; food- 

 supplies are checked, the Y.I*, is plugged off at the chalasa, and the scar of the hihtsa 

 is protected in continuity with the testa-layers. The seed loses water, and dries off. 

 often shrinking to half its volume : successful perennation is usually associated with 

 extreme conditions in this respect Great range of variation obtains: cf. mtnote 

 seeds of Willow, and massive seeds of Horse-chestnut, with little vitality if dried. The 

 general mechanism may be masked by new departures ; cf. fleshy fruits aa Apple, cut 

 off with seeds enclosed to rot on damp ground; Fig, whole inflorescence cot away, 

 edible, rating, or animal-dispersed. The abstraction of an air-dry, wind-borne seed 

 is the primary condition for Land Flora. 



Life of the Seed : Metabolism is reduced to a minimum, but some sort of 

 katabolism must continue : little evolution of CO, can be checked in air-dry seeds, 

 or absorption of oxygen, katabolism is intramolecular ; le. changes are too small to 

 give an appreciable gaseous exchange, and life may be prolonged a considerable time 

 at expense of stored material No seed has been known to live 1 50 years (few 1 00-50). 



II 



