In the willow and the milkweed an aril is developed in the form of a tuft of hairs. (In the willow it is an outgrowth of the funicle, = stalk of the ovule, and is called a funicular aril; while in the milkweed it is an outgrowth of the micropyle, = the open end of the ovule, and is called a micropylar aril.)
664. Increase in size during seed formation.—Accompanying this extra growth of the different parts of the ovule in the formation of the seed is an increase in the size, so that the seed is often much greater in size than the ovule at the time of fertilization. At the same time parts of the ovary, and in many plants, the adherent parts of the floral envelopes, as in the apple; or of the receptacle, as in the strawberry; or in the involucre, as in the acorn; are also stimulated to additional growth, and assist in making the fruit.
665. Synopsis of the seed.
| The seed. | Aril, rarely present. | |
Ovular coats (one or two usually present), the testa. | ||
Funicle (stalk of ovule), raphe (portion of funicle when bent on to the side of ovule), micropyle, hilum (scar where seed was attached to ovary). | ||
| Ripened ovule. | Remnant of the nucellus (central part of ovule); sometimes nucellus remains as Perisperm in some albuminous seeds. | |
| Endosperm, present in albuminous seeds. | ||
Embryo within surrounded by endosperm when this is present, or by the remnant of nucellus, and by the ovular coats which make the testa. In many seeds (example, bean) the endospermis transferred to the cotyledons which become fleshy(exalbuminous seeds). | ||
666. Parts of the ovule.—In [fig. 401] are represented three different kinds of ovules, which depend on the position of the ovule with reference to its stalk. The funicle is the stalk of the ovule, the hilum is the point of attachment of the ovule with the ovary, the raphe is the part of the funicle in contact with the ovule in inverted ovules, the chalaza is the portion of the ovule where the nucellus and the integuments merge at the base of the ovule, and the micropyle is the opening at the apex of the ovule where the coats do not meet.
Fig. 401.
A, represents a straight (orthotropous) ovule of polygonum; B, the inverted (anatropous) ovule of the lily; and C, the right-angled (campylotropous) ovule of the bean. f, funicle; c, chalaza; k, nucellus; ai, outer integument; ii, inner integument; m, micropyle; em, embryo sac.
Comparison of Organ and Member.