FUNCTIONS OF PLANTS. 25 



presence : 1. of "substances containing carbon and nitrogen, and capable of 

 yielding these elements to the growing plant ; 2. of water and its elements ; 

 and 3, of a soil to furnish the inorganic matters, which are likewise essential 

 to vegetable life." (Liebig, p. 4.) A summary view of the whole process 

 of vegetable development is thus given by Professor Henslow : " Plants 

 absorb their nutriment by their roots; this nutriment is then conveyed 

 through the stem into the leaves ; there it is subjected to a process by which~ 

 a large proportion of water is discharged ; the rest is submitted to the action 

 of the atmosphere, and carbonic acid is first generated and then decomposed 

 by the action of light. Carbon is now fixed under the form of a nutri- 

 tive material, which is conveyed back into the system ; and this material 

 is farther elaborated for the development of all parts of the structure, and 

 for the preparation of certain secreted matters which are either retained 

 within or ejected from the plant." (Descriptive and Physiological Botany, 

 p. 176.) This short passage comprehends the essence of all that can be said 

 on the subject of vegetable development ; but for the purposes of horticul- 

 ture it will be useful to go more into detail and to consider vegetable de- 

 velopment under the form of germination, growth, function of the leaves, 

 action of the flowers, and maturation of the fruit and seed. 



102. Germination. The seed containing an embryo plant, its develop- 

 ment is effected by its being placed in suitable circumstances for that purpose. 

 These are moisture, warmth, the absence of light, and contact with air ; to 

 which may be added, with a view to cultivation, the presence of soil. The 

 undeveloped seed is principally composed of concentrated carbon, and in the 

 act of germination, this carbon, by the absorption of water, is converted into 

 mucilaginous matter, which is decomposed and rendered soluble by the 

 oxygen of the atmosphere. Thus it appears that the first act of germination 

 is to reverse the process of maturation ; and hence, the reason why all seeds, 

 if sown fresh, when they are nearly ripe, will germinate more speedily than 

 when fully ripe ; and when fully ripe, sooner if sown immediately than if 

 kept for months -or years. The soluble mucilage of the cotyledons supplies 

 the embryo plant with nourishment till it is able to extract food from the 

 soil, after which it absorbs this food from the soil, by the points of its 

 radicles./ Seeds will not germinate without the presence of oxygen. In 

 nitrogen, or in carbonic acid gas, if moistened with water, they will 

 swell, but not vegetate. Hence seeds excluded from the atmosphere and 

 from water, may be preserved from decay for an indefinite period ; but it 

 does e not follow that during the whole of this period they will retain 

 their vital principle. The presence of light is not only unnecessary to the 

 germination of seeds, but injurious, and hence in horticulture they are 

 always more or less buried in the soil, generally to a depth equivalent to 

 the diameter of the seed. The temperature required to germinate seeds, 

 varies from 32 to 80 or 90 ; and some seeds, such as those of the Robima 

 Pseud-acacia, and of some species of Australian Acacias, may be immersed in 

 water at the boiling point, and kept for some minutes in it without destroy- 

 ing vitality. The seeds of no plant will vegetate under 32, because below 

 that degree water freezes, and consequently could not be absorbed by the 

 tissue of the seed. The common annual grass (Poa annua) will vegetate at, 

 or very slightly above, that temperature, as will the Chickweed, (Alsine 

 media,) the common Day Nettle, (Lamium rubrum,) and various others. The 

 process of malting barley is exactly the same as that of germinating a seed. 

 By moistening the barley, it swells, the starch of the cotyledon is changed 



