London Horticultural Society and Garden, 621 



illustrating an experiment to prove the different proportions of oxygen 

 given out by plants under different circumstances. When the roots of a 

 plant were immersed in water strongly impregnated with carbonic acid gas, 

 pure oxygen was given out by the leaves; but when the roots were in 

 distilled water, and the leaves were surrounded by oxygen, no oxygen 

 was given out. 



The extreme importance of air to leaves having been proved, it is neces- 

 sary to say a few words on its usefulness to roots ; and this is proved by 

 trees always nourishing most when some portion of their roots is near the 

 surface. Even those plants which send down a tap root have always 

 some fibrils to spread out near the surface, so as to enjoy the influence of 

 the air. In pots, plants show the same propensity, the fibrils always 

 extending themselves as closely as possible to the porous sides of the pot. 



Lecture VI. Functions of the Leaves ; Importance of Light and Air 

 to Plants ; Colour ; Succulent Plants ; Wood and Iron Hot-houses ; Cuttings ; 

 Layers; Grafting; Inarching; Pruning; Bulbous Plants ; Management of 

 Plants in Rooms, in Green-houses and Hot-houses, in Pits ; Latitude not 

 always a Criterion for Climate ; Bottom Heat; Conclusion. — The professor 

 began by stating that he had, in a former lecture, left unfinished one of the 

 most interesting and most important subjects in the whole range of vege- 

 table physiology : he alluded to the functions of the leaves. He had 

 before stated that leaves absorb carbonic acid during the day, and oxygen 

 during the night, forming carbonic acid in proportion to the oxygen 

 they have absorbed ; that they decompose their carbonic acid during 

 the day, setting free the greater part of the oxygen it contains, and 

 retaining the carbon, which appears to afford them food ; and that this 

 process is necessary to their vegetation, though the exact manner in which 

 it acts is at present unknown to botanists. The importance of light and 

 air to plants is well known. When unassisted by these agents, plants lose 

 their colour, and are deprived of many of their properties. Colour is thus 

 evidently produced by the absorption of carbonic acid gas; and the colour- 

 ing matter may be detected by a powerful microscope, lodged in the cellular 

 substance of the leaf. How this colour is formed, and why it assumes 

 different tints in different plants, are, however, questions which it is at 

 present impossible to decide. The secretions of plants depend upon light, 

 and their flavour and nutritious qualities are materially altered by their 

 exclusion from it. The importance of this knowledge to a practical horti- 

 culturist is proved by the fact, that sea-kale, so well known as a wholesome 

 and palatable vegetable, is not eatable in its original state ; and that any 

 part of the cultivated plant, if accidentally left exposed to the action of the 

 air and light, becomes tough, and so strong in flavour as to be extremely 

 unpleasant to the taste. Celery, also, in its native state, is poisonous ; 

 and it is only the parts that are blanched that are perfectly fitted for the 

 table. Though colour is generally supposed to depend principally on the 

 plant's being exposed to the light, some portion of colouring matter 

 appears to be occasionally absorbed by the root. This colouring substance 

 is, however, never a deep green. Red and yellow, as may be seen in 

 forced rhubarb, &c, are the most common hues. Succulent plants are less 

 susceptible of the influence of light than any others. As they are always 

 natives of hot countries, nature, to prevent the danger they would be 

 exposed to from excessive evaporation, has provided them with leaves 

 almost destitute of pores ; and the moisture they absorb by their roots 

 thus remains for the nourishment of the plant. [See p. 234.] It is for this 

 reason that cactuses, mesembryanthemums, and other plants of a similar 

 description, require very little water when kept in pots. Scarcely any 

 carbon is found in plants grown in the dark. Many experiments have been 

 tried to show the stimulus afforded to vegetation by light; trees of the 



