THE NUCLEUS. 135 



form the new plant ; and, in most instances, one or more masses of albumen destined to 

 supply food to the newly forming plant. 



The direction of the embryo in the seed varies as greatly as that of the nucleus in 

 the ovule, and is always determined by similar means viz., the position of the chalaza, 

 micropyle, and raphe. The terms employed to designate this relation are similar to, 

 but not identical with, those given at page 128 in reference to the ovule. Thus antitropal 

 in the seed corresponds with orthotropal in the ovule, the sacs of the ovule not being 

 inverted, but the embryo inverted with respect to the seed, as in the Stinging Nettles. 

 Orthotropal in the seed, as in the Apple, is the anatropal of the ovule ; amphitropal in 

 the seed, that of camphylotropal in the ovules, as in the Mignonette, and have both 

 apex and radicle next to the hilum ; and last, heterotropal in the seed, is the amphi- 

 tropal or semi-anatropal of the ovule, and they lie across the seed. In the antitropal 

 and amphitropal forms there will be neither raphe nor chalaza; whilst in the ortho- 

 tropal and heterotropal varieties both these parts will be present. 



The above is indicative of the relation which the embryo bears to other parts of the 

 seed ; but there is also a relation which the whole seed has to the fruit of which it 

 forms a part. The seed is termed ascending, when the direction of its apex is that of 

 the apex of the fruit ; descending, when the contrary, or towards the base of the 

 fruit ; centrifugal, if towards the sides ; and centripetal, when towards the axis of the 

 fruit. 



The albumen varies greatly in quantity, as may be seen by contrasting the split Pea 

 with the white of the Cocoa-nut. It also offers great diversity in solidity, from a mass 

 of jelly-like consistence, to the hardest ivory, as in the Ivory Nut (Fig. 38) in its dried 

 state. It is not present in all seeds, and in many is so minute in quantity that the 

 microscope alone can detect it. Wherever it exists, it immediately surrounds the 

 growing point. Its structure is cellular, as shown in Figs. 37, 38, 39, and others, 

 as may be readily proved, by placing a very thin portion of a green pea under the 

 microscope. When it is met with in the embryo sac (page 129) it is called Endosperm 

 (within the seed) ; and when it constitutes the nucleus it is known as the Perisperm 

 (around the seed). Sometimes it is placed near to the chalaza ; but it never occupies 

 the position of the membranes. In Dicotyledonous plants, as the Pea, 

 the seed readily separates into two halves, which proves that the mass 

 is divisible into two lateral and symmetrical portions, termed Cotyledons, 

 or seed-leaves (Fig. 262). In Monocotyledonous plants, as the Palms, the 

 albumen cannot be divided into parts, and hence the terms Monocoty- 

 ledon, or one seed-leaf (Fig. 264) ; and in certain plants it is so reduced 

 in quantity that the seed is termed Acotyledonous, or a seed without coty- 

 ledons. These terms are of great moment, and of constant employment, 

 since the two former correspond to the exogenous and endogenous divi- 



FiR. 264. The 8 i ons o f plants referred to at page 81, et sea. All flowering exogens, or 

 Monocotvledo- 



nous seed of nearly so, are Dicotyledons ; all endogens, or nearly so, Monocotyledons ; 

 the Grass after and all fl owe rless plants, Acotyledons. The arrangement of the parts 

 germination . T_ i 



has begun. in the embryo varies in the classes just mentioned, as might be 



C ' ^imnule 1 '' ex P ec * e ^> when in germination one puts forth no seed leaves, another 

 r, radicle. only one, and a third two. The direction of the Cotyledons is usually 

 straight ; when two or more exist, they are placed face to face. They are said to be 

 incumbent when they are folded with their back upon the radicle, but accumbent when 

 their edges occupy that position. 



