34 



NTRODUCTION. 



The embryo or rudiment of the new plant is produced within the sac of 

 the amnios, which is mostly absorbed during its growth, but sometimes 

 remains in the form of a bag, called the vitellus. Where no albumen is pre- 

 sent, the whole substance of the nucleus is 

 Fig- 3i. absorbed, and the embryo occupies its place. 



The embryo consists of one or more lobes, 

 called cotyledons, which surround the ter- 

 minal bud called the plumule, and the por- 

 tion which becomes the root, termed the 

 radicle. In some rare cases the embryo is 

 destitute of cotyledons; these plants are 

 called acotyledonous ; where there are two 

 cotyledons they are dicotyledonous; this is 

 seed of onion, a a Albumen, b Embryo, the case in all the exogenous plants ; where 



there is but one cotyledon, they are mono- 

 cotyledonous, as occurs in the endogenous plants. 



The embryo is either straight, or curved, or spiral. Its position as regards 

 the albumen, when this is present, is usually in its axis ; but it is some- 

 times eccentric, as in the palms ; or external, as in Jndian corn, &c. ; where 

 it is external, or nearly so, and curved round the albumen, it is peripheric. 

 When it is so bent that the radicle is placed against the edges of the cotyle- 

 dons, the latter are said to be accumbent, and when the radicle rests against 

 the back of one of them, they are termed incumbent. The radicle always 

 points to the micropyle ; and the apex of the cotyledons, except when spiral, 

 to the chalaza. 



VEGETABLE PHYSIOLOGY. 



Propagation of Plants. 



Germination. A perfect seed may be considered as a plant, whose vital 

 energies are in a dormant or latent state, requiring the concurrence of cer- 

 tain circumstances, to call them forth and to produce such a condition as to 

 fit it for an independent existence. Seeds retain this latent vitality for periods 

 varying much in different plants; some losing it in a few months, whilst 

 others retain it for an indefinite period. The conditions requisite for germi- 

 nation, are, moisture, the presence of air, the absence of direct light, and 

 warmth. Moisture probably acts in several ways ; it softens the integuments, 

 pervades and softens the nutritive matters, and thus brings them to a fit state 

 to be absorbed by the embryo ; conveys in solution nutritive particles from 

 other sources ; and by its own decomposition affords two of the most impor- 

 tant ingredients of vegetable bodies. The presence of air is necessary, be- 

 cause the oxygen contained in it causes a change in the starch contained in 

 the albumen or cotyledons, and converts it into a semi-fluid substance consist- 

 ing of sugar and gum, and combines with some of the carbon, forming car- 

 bonic acid which escapes, whilst the proportion of oxygen and hydrogen are 

 augmented. The absence of light is favourable, as its presence has a ten- 

 dency to produce an opposite change, the accumulation of carbon. Heat 

 promotes the necessary chemical changes, and assists the moisture in acting 

 on the hard parts of the seed ; and also probably acts as a stimulus to the 

 absorbents of the embryo. The degree of heat required is much the same in 

 similar species, but varies greatly in different plants ; thus the seeds of some 

 plants will germinate at a temperature near that of 32°, whilst those of others 

 require a heat of 90° to 110°. Seeds germinate most freely in spring and 



