Growth of the Embryo : The first division of the Zygote-nucleus (oospore) 

 determines the orientation and polarity of the new individual ; and of the two cells 

 thus produced, the inner may be termed the embryo head-cell, the other is utili/ 

 ntspewor. Division of the latter gives a short filamentous series of a few units, with 

 the effect of pushing the head-cell down into the cavity of the embryo-sac, \\1 

 can absorb food-material from the developing endosperm. The head-cell gives the 

 bulk of the new plant, dividing symmetrically to 2-4-8 (or/awAstage), and then 

 becoming 2- and 3-layered by the addition of periclinal walls. 



Differentiation of regions in the embryo ' head '-portion follows ; 2 lateral lobes 

 grow out (cotyledons), the apex of the stem is localized between them ; while the apex 

 of the primary root is marked out at the opposite pole, at the junction with the 

 suspensor ; the last cell of the suspensor being required to fill in the curve-pattern. 

 Further elaboration of these growing-points follows the plan of the same regions in 

 the adult. The primary root (Radicle) is thus orientated pointing to the micropyle. 



Differentiation of Testa: The ovules with included endosperm and embryo grow 

 to the full seed-size; the ovule with its contents constituting the seed-stage. An 

 embryo being formed and differentiated, all subsequent modifications have reference 

 to the manner in which it can be (i) nourished, (2) protected from desiccation, 

 (3) ultimately separated from the parent plant, with (4) chances of being dispersed. 



(1) Increase of volume in the ovule may be considerable; ovules are commonly 

 i mm., often less, and seldom over 5 mm. ; seeds, as in Coco-nut, may be 6 in. 

 long, with the same general proportions and parts. Nutrition is effected by 

 increased formation of V.B. in the outer nucellus-layers. 



(2) The outer layers of the nucellus, as also integument, commonly add the effect of 

 sclerosed cell-units, in utmost variety, as protective testa ; the differentiation 

 being continued over the entire surface of the ovule, without regard to special 

 regions. The chalaza remains the base of food-supply. 



(3) With a view to separation from the parent plant, the chalazal region is normally 

 sealed across by a special chalazal-plug, more or less similarly scleroscd, and 

 an abscission-layer is prepared at the At/urn, cutting across the funicle or base 

 of attachment 



(4) The ovule with its testa-layers, remains of nucellus, endosperm, and embryo, at 

 hatever stage of growth it may have attained, is cut away as the Seed. 



CAPSELLA (Shepherd's Purse), a small Crucifer weed, affords a convenient 



example of early embryology; the growth of the embryo being followed without 



section-cutting. The ovary is syncarpous, of 2 carpels; the 2 loculi containing 



io-i2 ovules each : on picking out these ovules from a green fruit, in a drop of water 



on a slide, and adding a drop of KOH and a coverslip, a gentle pressure will cause 



the embryos to ' jump out ' of the turgid embryo-sac, and lie free in the liquid. A series 



of stages may be made out, beginning with the oldest and most easily obtained : 



X. From full-grown fruits with seeds nearly ripe, a large embryo is obtained with 



well-differentiated radicle, and two green cotyledons bent sharply on the 



radicle (i mm.) in the campylotropous ovule. 



IX. From younger fruits, 5 mm., a straight embryo, with two cotyledonary lobes, 



and radicle-apex with relic of suspensor, 360 p over all, cots, half the length. 



VIII. Smaller fruits show embryos much smaller, with triangular or bilobed head 



as the first indication of cotyledons, suspensor-filament of 8 cells with enlarged 



bladder-like basal unit (150 /x). 



VII. Embryo with globular multicellular head (50 /x diam.)_on a similar suspensor- 

 filament: earlier stages require to be neutralized with HA to bring out cell-walls. 

 VI. Head-region 40 ^ diam., differentiated in 3 layers suggestive of embryonic 



men stems, 



V. Head-region 30 /t, 2-layered by first periclinal walls, and a few anticlinals. 

 IV. Globular octant-stage, 20-30 p. diam., head-region marked with conspicuous 

 cross : suspensor-filament of 5 cells, 18 p. diam., and basal bladder-cell 100 /z. 

 III. Stage of quadrant-division (4 nuclei only), and first longitudinal wall (2 -celled). 

 II. Head unicellular (20/1 diam.), suspensor of 3-4 cells, lowest enlarged. 

 I. Left for earlier stages not obtainable by this method. 



Note : each stage adds a new construction-factor to the organism. Development 

 is largely on mechanical lines until the outgrowth of the 2 cotyledons. 



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