EMBRYO-SAC. 



[ 226 ] 



ENCALYPTA. 



Bibl. of tlie articles Ovule, Seed, &c., 

 above referred to. 



EMBRY^O-SAC, of Plants.— A cell 

 which becomes enlarged into a sac in the 

 substance of the upper part of the nucleus 

 of the ovule or rudiment of the seed. In 

 the cavity of this are developed the germinal 

 vesicles (PI. 38. figs. 3,4, 5), one of which 

 (occasionally more), after fertilization, gives 

 origin to the Embryo. The most common 

 condition of the embryo sac is that of a large 

 cavity excavated in the nucleus, bounded by 

 its own cell-membrane, and containing abun- 

 dant protoplasm, and subsequently germinal 

 vesicles and endosperm-cells (see Ovule). 

 Not unfrequently, however, it becomes de- 

 veloped into diverse saccate processes, either 

 pushing their way through the substance of 

 the nucleus in variable directions {Scrophu- 

 lariacecB, &c.), or emerging from the micro- 

 pyle, coming to meet the pollen- tube {Vis- 

 cum), or even so much developed externally 

 that the embryo is formed and perfected 

 altogether outside the nucleus {Santalum). 

 These and other conditions are further de- 

 scribed under Ovule. When the germinal 

 vesicle is fertilized, and is undergoing deve- 

 lopment to produce the embryo, the embryo- 

 sac often becomes completely filled with 

 endosperm-cells, at first free, but afterwards 

 adhering together through their crowded 

 condition. These may persist and form an 

 endosperm to the seed, as in Nuphar, where 

 there is an additional episperm formed out- 

 side the embryo-sac from the substance of 

 the nucleus. More frequently, both in albu- 

 minous and exalbuminous seeds, the endo- 

 sperm originally existing inside the embryo- 

 sac becomes absorbed through the pressure 

 of the growing embryo, and altogether obli- 

 terated, the embryo gradually filhng up the 

 cavity, and by further expansion obliterating 

 all trace of the embryo-sac itself. (See Al- 

 bumen, of Seeds.) 



In the Coniferse, the embryo-sac, origi- 

 nally formed by the excessive expansion of 

 one of the cells near the apex of the nucleus, 

 becomes subsequently filled up by cellular 

 tissue, in the upper part of which become 

 developed the bodies called corpuscula, each 

 of which possesses a kind of secondary em- 

 bryo-sac of its own, in which the germinal 

 vesicles are developed (see Gymnospermia). 



The term embryo-sac might also be applied 

 to the large cell at the base of the archegonia 

 of the Ferns, Lycopodiace^, Mosses. 

 (See under these heads.) 



Bibl. See Ovule and Gymnospermia. 



EMY'DIUM, Doyere {Echiniscus, Schultze) . 

 — A genus of Arachnida, of the order Colo- 

 poda, and family Tardigrada. 



Char. Head furnished with appendages ; 

 mouth conical, without appendages or ter- 

 minal sucker ; epidermis semisolid, present- 

 ing, especially on the upper surface of the 

 body, an evident annular division. 



E. testudo{F\Al.Qg.7). Reddish-brown; 

 body ovoid, opake ; snout conical, presenting 

 traces of division into three rings; head 

 indistinctly divided into three segments, 

 the first and third presenting short setiform 

 filaments supported upon very short tuber- 

 cles, the second with a palpiform, blunt, and 

 flattened appendage ; pharyngeal tube very 

 slender; styles straight; bulb without an 

 internal jointed frame-work; eye-spots small, 

 oval, simple, most visible at the under aspect 

 of the body ; trunk divided into four simple 

 rings, with spines and long filaments ; legs 

 three-jointed, each with large and strong 

 claws, the posterior pair with a kind of spur 

 also at the back part of the lower margin of 

 the second joint; movement excessively 

 slow; length, from the end of the extended 

 snout to the posterior border of the fourth 

 ring, 1-80". Found on the moss covermg 

 tiled roofs ; common. 



E. spinulosum \ 



E. granulatum J 



Bibl. Doyere, Ann. des Sc. nat. 1840. 

 xiv. p. 279. 



ENCALYPTA, Schreb.— A genus of Ca- 



Fig. 196. 



Fig. 197. 



Encalypta commutata. 



Fig. 196. A single plant. Magnified 5 diameters. 

 Fig. 197. The mouth of the capsule, showing the rudi- 

 mentary peristome. Magnified 10 diameters. 



