CHORIPETAL^ 325 



and seeds lie naked or exposed on the surface of the latter : 

 fertilisation is effected by pollen-grains which come into direct 

 contact with the micropyle of the ovule. 



The Angiosperms possess carpels which are hollow closed 

 structures, the ovules and seeds being developed within the 

 completely closed cavity or ovary of the carpels. In these plants 

 the pollen-tube must first pass through the tissues of the carpels 

 before reaching the ovule. 



4. As practically all farm plants belong to the Angiosperms it is 

 important to enter into greater detail in regard to the classification 

 of this sub-division of the Vegetable Kingdom. The following is 

 an outline of the arrangement and chief features of the Classes, 

 Sub-classes, and a few common Orders included in it. 



Sub-division 2.— ANGIOSPERMS. 



Class I. Dicotyledons. — In these plants the embryo has two 

 cotyledons and the floral-leaves are usually in fours or fives. In 

 a cross-section of the stem the vascular bundles appear arranged 

 in a single ring round a central pith and in perennial species 

 concentric zones or ' annual rings ' of wood are present, the 

 ' annual rings ' being formed by a cambium-tissue. The leaves 

 are generally net-veined. 



Sub-class I. Choripetalse. — The corolla when present is poly- 

 petalous. 



In some plants of this sub-class the flowers are imperfect; 

 either the corolla or calyx is absent or both parts of the 

 perianth are missing. 



(i) Flowers regular, hypogyvous, usually with a single green or 

 white perianth : fruit one-seeded. 



Order. Cannabacea (see p. 331). 



Order. Polygonacem. — Flowers small with a perianth of three 

 to six free segments : stamens five to eight opposite the perianth 

 segments ; gynaecium of two or three united carpels, the ovary 

 generally triangular or oval in section, and containing a single 



