STRUCTURE, MORPHOLOGY, AND PHYSIOLOGY. 19 



with its micropyle turned downward and outward. It 

 possesses two integuments (Fig. 6, A, i lt t f ), the outer one 

 of which has very delicate walls and forms a conductive 

 tissue for the pollen-tube upon the anterior side. It 

 disintegrates soon after fertilization. 



The embryo-sac (Fig. 6, A, es) grows rapidly after fer- 

 tilization at the expense of the tissues of the endosperm, 

 so as to leave only 1-2 layers. At the same time it be- 

 comes filled with considerable albumen in whose cells, 

 the outer layer excepted, is deposited an abundant sup- 

 ply of starch. The ovule meantime grows rapidly in 

 length and later also in breadth, and finally fills the 

 cavity of the ovary so completely that any further 

 growth causes it to grow to the walls, and this fails to 

 happen only in a few cases. The embryo is at first a 

 many-celled club-shaped body whose vegetative point is 

 situated in a lateral depression ; the portion of the em- 

 bryo above this depression becomes the cotyledon, which 

 farther expands, becoming shield-shaped. Then the 

 borders of the depression in which the vegetative point is 

 situated rise up in the form of a collar, the beginning of 

 the first sheath-like leaf, hypophyllum, and opposite 

 to it, later on, is developed the first foliage leaf. The 

 initials of the main root lie deep within the lower half of 

 the young embryo, so that the surrounding tissue grows 

 with the root for some time until the latter separates 

 itself from the tissue around it by forming a cleft. But 

 the entire radicle is still imbedded in it, and not earlier 

 than the seed germinates the root first breaks through 

 this tissue, called the coleorhiza or root-sheath (compare 

 c in Fig. 6, L, N). Upon the sides of the main radicle 

 2-4 secondary radicles are occasionally situated (iv t , iv y 

 in Fig. 6, F], and on the other end of the axis the begin- 

 nings of the foliage leaves increase in the bud to 3 or 4, 

 so that the germ of the grass at the maturity of the seed 

 has reached a high degree of development. 



Fruit and Seed. The fruit of most grasses is a cary- 

 opsis the thin pericarp of which is grown fast to the 

 seed. The pericarp here answers the purpose of the 

 testa, which is only very feebly developed ; it gener- 



