CUCURBITA 



591 



layers form a true spongy tissue of thin-walled cells, and the 

 innermost layers of this zone are chlorophyllose. The inner 

 epidermis (5) is a layer of small thin-walled cells. (Fig. 308, A.^ 



Underlying the seed coat is a zone of perisperm or nucellar tissue 

 about six layers in thickness. The cells are thin-walled, except 

 for the outermost one which is epidermal with a definite cuticle. 

 The endosperm consists of a layer of thick-walled cells which have 

 granular contents and large nuclei. The embryo has a short, 

 blunt hypocotyledonary axis from which two flat, somewhat 

 fleshy cotyledons are diverged. A very short conical growing 

 point constitutes the epicotyl. 



Development of the Seedling. — Owing to the shape of the 

 seed, it lies flat in planting. Under favorable conditions, the 

 primary root protrudes between the halves of the seed coat at 

 the end of two days and extends downward at right angles to the 

 seed. The root grows very rapidly, and may be 3 cm. in length 

 by the end of the third day. At this time the peg begins to develop 

 as a small, lateral, parenchymatous outgrowth in the angle formed 

 by the base of the horizontal hypocotyl and the vertically oriented 

 root. By the fourth day, the peg is enlarged considerably, in 

 some varieties extending part way round the axis; and just below 

 it, the primary root produces many lateral roots. The hypocotyl 

 begins to elongate upward while the cotyledons still remain within 

 the seed coat. This soon splits, since its lower half is held in 

 place by contact with the lower surface of the peg while the upper 

 half is forced open by the growth of the arching hypocotyl. 

 (Fig. 309, A-C.^ In some instances, the lower half of the seed 

 coat slips from the peg, and the seed coat is carried above ground, 

 where it is later shed as the cotyledons enlarge. 



Eventually, the cotyledons are freed from the seed coat, emerging 

 from the soil; and about the sixth day, the axis becomes erect. 

 The seedling phase is usually completed in seven or eight days, 

 when the primary structures are differentiated, and the reserve 

 foods stored in the cotyledons have been largely utilized. The 

 young plant is then independent, the outspread cotyledons are 

 photosynthetic, and there is a well-branched root system. The 

 growth of the epicotyl is relatively slow during germination; 

 but the first foliage leaf finally develops very close to the coty- 

 ledonary plate, since the first internode is extremely short. (Fig. 

 309, D, £.) 



