When the branches develop themselves and consolidate, the movement of the sap becomes slower; it is sometimes roused towards the end of summer, when, the spring having been premature, the materials which the plant has elaborated for the vegetation of the following year have been set to work before their time. After the fall of the leaf, and when the approach of winter lowers the temperature, the movement of the sap is stopped entirely;the tree arrives by little and little at a state of almost absolute repose: this is not death, but life, which awaits its reawakening.

It thus appears that all plants in a healthy state must be so situated, as to be able to absorb from the soil surrounding them the elementary bodies which constitute sap, namely, carbon, oxygen, hydrogen, and nitrogen, which are capable of forming a number of secondary combinations; as well as those which are necessary to the growth of certain families. The principal form in which these — bodies are absorbed is, as water, carbonic acid, and ammonia.

Water is supplied to plants in the form of rain and vapour; according to Schleiden, by far the greater part being due to the latter process, Carbonic acid, which supplies plants with the carbon of their tissues, is supplied by the respiration of man and animals: by combustion, and by- the decomposition of saccharine matter, volcanic action, and hot springs. I

The sources of ammonia are nearly the same as carbonic acid: the secretions of animals, the combustion of wood and coal, and the decomposition of nitrogenous matter.

The constituents of sap in special plants are the metallic oxides, as potassium, sodium, calcium, and magnesium. How these various substances permeate through the tissues of plants, is, to a certain extent, a mystery.:

On THE GrRowTH OF VEGETABLES.

Vegetables increase by means of the nourishing materials elaborated by the descending sap. It would be entering upon a long and difficult task to take into consideration all the points which botanists have made concerning the growth of vegetables. We must, therefore, limit ourselves here to the consideration of what actually comes under our own knowledge in the growth of trees. Trees are elongated by the development of their terminal shoots. We observe the intervals between the points of the insertion of the leaf, which at first are very short, gradually become longer according to the greater degree of development through which they have passed.

But how do trees increase in circumference? This demands @ closer examination. If we study the internal structure of a branch of one year old in one of our forest trees, we shall find that towards the end of the year the branch is arranged in the following manner. There is @ pith, a fibro-vascular circle, furnished with air-vessels on its internal face, with straight medullary rays traversing the ligneous _ circle, losing themselves in the bark, which is composed of an epidermis of ruber, of the herbaceous envelope, and of liber. But between the ligneous and cortical system we can distinguish the presence of a special zone formed of very delicate cells, with walls soft and transparent, which in spring-time are bathed in liquid supplied abundantly by the descending sap, to which botanists give the name of cambium. This last layer is of the utmost for we can perceive that in the course of the importance, second year, by the process of vegetation, this intermediate zone, which is situated between the cortical and ligneous system, and

has received the name of the generating zone, becomes the seat of

a double formation, the one cortical, the other ligneous. Fig. 137, which we have given to show the different elements of the stem of a tree, serves also to show its mode of development. That part included in the lines marked 1, represents the wood and bark of the first year, that marked 2 the wood of the second year. The generating layer separates these two elements, and is indicated by the letter ¢. Cells, fibres, and vessels result from the transformation of the delicate elements of this generating tissue. In other words, at this point the transposition of caméium, or bark from without to within, and of wood from within to without, takes place. The medullary rays are continued without interruption or modification across the new beds, forming itself anew, and, without

VP RMN VP FB TEM M

Fig. 137.—Horizontal andt ion ofthe trunkofanElm n

being in connection with the pith, prolonging itself, in short, to the bark. What passes in the second year is repeated during the third and fourth. A very useful consequence results from this. We are enabled by observing certain characters to distinguish the Successive layers which form themselves year by year. The age of a tree is thus, so to speak, inscribed upon its own sutface. In the oak, for example, it is very easy to distinguish the annual layers. The transformation of the generating zone in the wood 12

continues from spring to autumn, and consequently under climatic influences very different. The great vessels are formed at the season when the sap is most active in its circulation;then come vessels of smaller calibre and much more numerous. Towards the end of the year, when vegetation is less active, ligneous fibres are formed. It follows from these differences between the wood of spring and of the autumn, that it is very easy to distinguish the passage from the former to the latter, and that the different annual formations appear consequently as so many concentric zones on the horizontal section of the tree. To continue, it is by the operation of the generating zone that the tree increases im circumference. If the annual growth of a tree were equal during all the time of its vegetation, there would be no line of demarcation between each annual period, and consequently no concentric zones. It is thus with trees in some hot countries, where vegetation is neither checked nor accelerated by change of temperature. e age of a tree is not there, as in temperate countries, to be found inscribed upon its stem. | We must here, however, offer two important remarks. The new wood alone participates in the nature of the exterior parts of the ligneous circle; the medullary sheath is not renewed. On the other hand, new cells are constantly formed by means of the generating zone, in default of which the growth would cease as soon as all the cells of this zone were transformed into the elements of new wood and bark.

a Reet? rn RAN 9

Fig. 148.—Flower of Raftlesia Arnoldi.

Tue FLower.

We have surveyed with wonder the wisdom and power of the Creator as displayed in the different organs and appliances just passed in review. We have admired the roots, with their innumerable tufts of fibre, which by a marvellous faculty, scarcely explicable to us, imbibe the liquids contained in the earth, and convey the nourishing fluid into the tubes of plants; the stems and branches which support the plant in mid air; the leaves, organs at once of respiration and evaporation; the vessels, so variable in form; the breathing pores (stomata), the cells; ir short, all the appliances, all the living mechanism by means of which the vegetable functions are carried on; all tending to the production of flowers, which, in their turn, live only for the production of fruit;while the fruit itself only exists for the purpose of developing the seed, that ultimate end and essential design of vegetation; for Nature seems to concentrate all her efforts with a view

to the reproduction of the individual, and consequent preservation of the species. But what is a flower? What definition shall we give of a flower, that can pretend to exactness, and at the same time be framed in scientific terms? A rigorous definition of the flower is more difficult than one would think. It was a saying of Linneus that minerals grew, plants grew and lived, animals grew, lived, and felt;but this is now known to be altogether incorrect, for certain plants not only grow and live, but give every indication of feeling, witness the Mimosa, or Sensitive plant, which closes its leaves on being touched, and some of the most obscure plants among the Conferve, which move about by the action of their own e7/ia, or hairs, until they have found a resting-place for themselves. He would, in short, be a rash man who should attempt a definition of plant or mineral in these enlightened days. Such, however, was not the opinion of Jean Jacques Rousseau, the celebrated philosopher of Geneva, who was indebted to the study and cultivation of botany for some of the happiest hours of his life. He has left, in his “Letters on Botany,” a book full of interest and sound science, in which he thus expresses himself on the definition which can be given of a flower:—

“Tf I resigned my imagination to the pleasing sensations which this word seems to call forth, I should write a paper agreeable,

perhaps, to sentimental shepherds, but very unsatisfactory to botanists. Let us put aside for a moment the vivid colours, sweet odours, and graceful forms of flowers, and try, in the first place, to understand the organised being which unites these attributes. Nothing at first sight appears easier. Who thinks he requires to be taught what a flower is? ‘When no one asks me what is time,’ said St. Augustine,‘I know it very well; but I do not

know it when I am asked.’ One might say as much of a flower, perhaps of its beauty even, which is the prey of time. I am presented with a flower, and I am told, ‘Here is a flower.’ This is showing it to me, I confess, but not defining it; nor will this inspection enable me to decide, in the case of any other plant, whether what I see is, or is not, a flower; for there are multitudes of vegetables which have in none of hair parts the apparent colour which Ray and Tourncfort have introduced into their

definitions of a flower; and yet these vegetables bear flowers not less real than those of the rose-tree, although much less apparent.” Nevertheless, although the definition of the flower appeared to Rousseau so surrounded with difficulties, he does not hesitate to propound the following:— “ The flower,” he says, “is a local and temporary part of the plant, which precedes the fecundation of the germ, in or by means of which part the fecundation is effected.” This was an unexceptionable definition, which was scarcely modified a century later by Moquin-Tandon, when he said, “A flower is that temporary apparatus, more or less complicated, by means of which-fecundation is effected.” The flower, then, is an apparatus composed of two envelopes, the calyx, the corolla, and the essential organs proper to ensure the

Fig. 139.—Section of the Foxglove. Fig. 140.—Section of the Cal i3V v7

reproduction of the plant, namely, the pistil, in which the seed is. afterwards enclosed, and the stamens, destined to fecundate the pistil. The position of the organs will be better understood by examining the following diagrams. Fig. 139 is a portion of the flower of the Foxglove (Digitalis purpusia), showing the stamens J a united with the corolla cor, p, the peduncle, or floral axis, c, the calyx, 0, st, s, the pistil. Fig. 140 is an ideal figure showing how the calyx c, the corolla cor, the stamens s, and the pistil p, are consolidated throughout their basal portions.

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