hood, quinine from the bark of the chinchona, and strychnine from the seeds of Nux vomica. Tea and coffee owe their well-known refreshing qualities to the presence of certain alkaloids in their cells.

The albuminoids are stored up in the protoplasm of active, living cells, and in many seeds in the form of albumen, &c. Other substances in small quantities are also stored up in the cells, as sulphur, iron, potash, silica, lime, phosphorus, magnesia, &c.

The conditions necessary for the growth of plants are soil, air, moisture, and heat above the freezingpoint. The quantity or degree required varies with the species, some requiring great depths of soil in which to develop their roots, others requiring but little, whilst in the case of misletoe and some tropical species of orchids soil is quite unnecessary, the plants growing on the trunks of trees. Some water-plants have the whole surfaces of their leaves and stems submerged, and therefore must be dependent on the water for their carbon. Many ferns and bog-plants require copious supplies of moisture, whilst succulent plants, as the Stonecrops and Houseleeks, delight in dry, stony places. With respect to heat, plants are widely distributed over the whole surface of the globe. The colder regions produce very few species, and those sparingly, but in hot, damp countries vegetation is very profuse.

CHAPTER III.

THE FERTILISATION OF FLOWERS.

IT is popularly held that the chief end of plants is to minister to man's sense of the beautiful in form and colour, but the recent investigations of scientific men should dissipate so presumptuous a theory. Man certainly does-unless his nature be very depraved— derive very great pleasure from the presence of flowers; but the purpose of the fine odours, the varied tints and exquisite forms of flowers is that the species shall be continued by the production of healthy seeds. How they are instrumental in effecting this we will explain.

A flower usually consists of four series of organs, differing widely in form and office, but all modifications. of the simple leaf. These are the calyx, consisting of leaves called sepals; the corolla formed of leaves called

Co....

Re

.Cou

-Sta

-Pi

FIG. 37.

petals; the stamens, and the pistil. The calyx and the corolla are known as the floral envelopes; the

stamens and pistil are the essential organs, because

An-

P

F.....

---Cal

FIG. 38.

they are necessary for the produc

-Cor tion of seeds. The calyx is usually green; the corolla generally conspicuous from its bright colours. In some plants the calyx or the corolla is entirely wanting, in which case the floral covering is termed a perianth, as in the Crocus and Lily.

The stamens consist of two parts, the filament or stalk (F), and the anther or pollen-case (An). The anther is the principal part of the organ, and is filled with a very fine powder like flour. When the anther is ripe, it splits open, and sets free this powder, or pollen as it is called. Examined under the microscope, this pollen is seen to consist of very minute cells filled with protoplasm (fig. 40). The pistil occupies the centre of the flower, and is surrounded by the stamens when these are present. It consists of three parts-the stigma or surface, the style or stalk, and the ovary at its base. The ovary contains the immature seeds or ovules, which require to be fertilised by the contents of a pollen grain before they can grow into perfect seeds. The style may be absent, and the stigma placed directly on the ovary. The stigma is either sticky, rough, or covered with long hairs, to retain the pollen grains. In some plants the stamens are found in one flower and the pistil in another; they are then termed

FIG. 39.

unisexual. If both are found in the same flower, it is bisexual. Sometimes, again, the staminate flowers

are produced on one plant, the pistillate on another. In this case they are known as diœcious plants. If both are found on the same plant, they are monæcious. To effect fertilisation, some of this pollen. must be conveyed from the anther to the stigma. Supposing a pollen grain adheres to the stigma, the moisture of that organ induces it to send out from its under side a very slender shoot, the pollen tube, which pierces the stigma and, increasing in length, penetrates the whole length of the style, finally entering the ovary, where it comes in contact with an ovule. As the result of this contact, the protoplasm of the pollen tube mingles with that of the ovule, celldivision takes place, and the seed gradually ripens.

F

.......pt

FIG. 44.

But before this can take place, certain difficulties must be overcome. If the pollen-bearing (staminate) flowers are on one plant, and the pistillate flowers on another, how is the pollen to be placed on the stigma ? Again, in those flowers which possess both stamens and pistil, the stamens open and discharge their

pollen in such a way that it cannot possibly fall on the stigma. In the Fuchsia flower of fig. 37, nothing seems easier, from the inverted position of the flower, than that the pollen should fall on the stigma; but in reality the position of this renders such a result more difficult, for the sticky surface can only be reached from below. In the Crocus the anthers open along their backs so as to discharge their pollen away from the stigma. There are many other remarkable contrivances in various plants actually to prevent the pollen being shed on the stigma.

The lesson of this is: it is not to the plant's benefit, but otherwise, that its own pollen should fertilise its seeds; and on experimenting with various plants, to ascertain the truth of this, it is found that all plants fertilised with pollen from another individual produce far more robust offspring than the plant which is fertilised 'by its own pollen. Indeed, in some plants the application of its own pollen to its stigma has had the effect of causing the pistil to shrivel up, and the flower to die. Why this is so we cannot tell; but it is beyond doubt a fact, applicable to the animal as well as the vegetable kingdom, that the offspring of individuals closely related are more or less sickly and weak. Seeing that Nature objects, as a rule, to self-fertilisation, we should naturally expect to find that she has taken means to ensure their cross-fertilisation.

The untiring observations of Darwin, Lubbock, and Müller have clearly shown that such contrivances are as plentiful as they are remarkable--in fact, some of them are absolutely startling in the delicacy and