but on the return of spring it germinates, and if potatoes are not near perishes. But if it is within reach of a potato tuber, it enters it; its mycelium penetrates the haulm, and coming out into the air develops zoospores and conidia, and thus repeats the life-history given by Mr. Smith. From the experiments made with it, it is clear that a wet season is the most favourable to its growth, and the potato has less chance of recovering from its attack; but it is to be hoped that with the information recently obtained with reference to it, scientific men may be soon in a position to give such advice as will enable agriculturists to cope with this potent enemy.

Fig. 22 represents another of these fungi which, like the Potato fungus, largely affect cultivators of the soil. This is the Oidium Tuckeri, or Vine Blight, which attacks the Continental vineyards and even the hothouse vines in our own country; frequently it is

FIG. 23.

FIG. 22.

the cause of great destruction. Recently it has been ascertained to be only one form of the fungus shown at fig. 23-Erysiphe, a species of which attacks the

pea. There is besides a very large number of these small moulds which attack various species of plants and fruits. A species of Peronospora (P. effusa) attacks spinach; another (P. gangliformis) is very injurious to lettuces; P. Schleideniana destroys young onions, and P. trifoliorum attacks the lucerne.

Exigencies of space compel us to close this chapter, though the number and variety of microscopic plants entitle them to a whole volume; but perhaps sufficient has been said to show the reader that a large amount of recreation and instruction may be obtained by a study of these low forms of plant-life.

CHAPTER II.

PLANT STRUCTURE AND GROWTH.

ALL plants commence existence as a single cell, like Torula or Protococcus, and it is only by the multiplication of these cells, the alteration of their form by pressure against each other, and their development into tubes, &c., that we get the wonderfully varied and beautiful forms of higher plant life. As we have seen, the lowest forms of plants are unicellular, but from these to the complex organisation of the forest trees there is a very gradual advance. In the same group as Protococcus-the Alga-we have plants with a larger number of cells, as Zygnæma and Oscillatoria. In the division of Fungi we have a similar advance from the simple form of Torula, through the moulds with strings of cells placed end to end, up to the mushrooms and toadstools. From the remarks in the previous chapter, it will be seen that plants are divided into two great groups, those that possess chlorophyll-green plants-and those that do not

fungi.

The green plants are again broken up into other divisions, according to the complexity of their organisation.

In Zygnema we have seen how the normal form.

of the cell is modified by contact at their ends, changing them from the oval or round form of Protococcus into an oblong. In higher plants, we may find them of every conceivable shape, but the modification is always due to this same cause-pressure; but we shall find it is exerted at the sides as well as at the ends.

Here in this diagram, at a, we have two cells scarcely touching at their sides, so that they retain

00

b

FIG. 24.

their simple form; but at b we suppose them to be pressed together, and this pressure alters their form, making flat the two sides that are pressed together. At c we suppose that a number of cells are placed side by side, and pressure exerted at both ends of the series, consequently they assume an oblong shape. At d they are pressed together from all sides, just as people are pressed together in a crowd, and the result is, the cells become many-sided (polyhedral), so as to occupy the least amount of space. Now in different parts of a plant we have these cells arranged and modified in different ways, and groups of cells arranged in a definite manner are termed tissues. The simplest form is cellular tissue, the cells composing it retaining much of the normal form, but frequently they become many-sided by the pres

sure of neighbouring cells. It comprises several varieties, of which the most

important is Parenchyma, in which the cells are so arranged as to leave spaces (intercellular spaces), through which air can circulate between the cells. It forms the principal portion of most plants, the

other tissues being embedded in it.

Another form of cellular tissue is called Sclerenchyma, from the cells being hard, owing to the thickening of the cell-wall. The "stones" of cherries,

plums, &c., are composed of sclerenchyma.

B..

C

W.

V

Pa

W.

Some of the cells assume a long, narrow, drawn-out form, like fibres or tubes, and from their appearing in compact masses among the parenchyma, they are known as fibro - vascular bundles. It is these bundles which form the midrib and "veins" of leaves and the hard part of the stem. They are made up of woodtissue, bast-tissue, and vascular tissue, packed together in wedge-shaped masses (fig. 27). On examining a cross section of one of these wedges with the microscope, we find it presents the appearance of fig. 28. o is the outer, i the inner end; B is the bast-tissue, C the cambium zone, W wood-cells, and V the openings of the spiral vessels and ducts.

FIG. 27.

FIG. 28.