and a headache is what we may fairly expect after sleep ing shut in with them in the same room. It is almost as bad to allow green boughs to remain there either, for, in the dark, even the green parts cease to purify the air, and begin like the others to manufacture carbonic acid, at the expense of course of their carbon, which thus by degrees is used up. Now, as it is the carbon which constitutes the solid fibres of plants and produces their green color, they soon become yellow and limp when deprived of light. You may, perhaps, have wondered why the gardener amused himself with smothering his poor lettuces by tying them up at top like a knot of "back hair," instead of letting them grow freely in the air and sunshine. It is, my dear, to make them more tender and delicate for you to eat; and those beautiful, crisp, yellow leaves, so delicious to the tooth, would have been green and tough, had they not slowly and quietly let out a great portion of their store of carbon in darkness during the last few days, before being gathered. Even without playing the gardener, you may assure yourself of his fact in a still more simple manner. Put a flat board upon the lawn and leave it there for three days; then take it up again, and you will find just where the board has prevented the light from reaching the grass, a yellow mark so distinctly traced as to be seen from the other end of the garden.

But to return to the sap, which we left undergoing a change from air and solar influences in the leaves. The ascending sap was to all appearance only clear water. When it returns from the leaves, charged with carbon, it is a thick juice having almost the consistency, and sometimes even the color of milk, and is possessed of properties altogether new. The most striking example that I can give you of the difference of the two states of

sap is the Euphorbia of the Canary Islands, whose di gestive or descending sap is a violent poison. When the natives of the country are accidentally pressed by thirst, they carefully remove the bark in which the fatal juice circulates, and are then able to refresh themselves safely by sucking the stem, which yields only the watery sap sucked from the ground, and as yet unaltered and harmless.

Each of these two saps, in fact, has its path distinctly traced for it: the first rises through the wood, the second descends through the bark, whence it is called descending sap. If you wish to satisfy yourself of this, fasten a rather tight knot of pack-thread round a young branch, and after a time you will see it pine below the knot and become swollen above it, an unanswerable proof that the nutritive juices flowed downward through the bark; for the wood inside the branch will have been uninjured by the strangling pressure. Remember this, my dear, when you are playing in the garden, and do not injure the bark of the young trees your father likes so much to see flourishing. It is by the bark that they are nourished, and you might even kill them by treating it too roughly.

And now I must show you how the nutrition is carried on, or, if you like better, how the tree grows by means of this descending sap. See: here is a fir tree, which has just been cut down to the ground. Now, if you like, I will tell you in a moment how old it is. I will even tell you the age of every branch, little and big ones both, without making a mistake in a single year; and you know as well as I do that I am no conjuror. You see these small circles so delicately drawn, as it were, upon the face of the sawn trunk, each wider than the last, as if they were composed of a set of tubes, of un

equal sizes, fitting exactly into each other. Now count them; and you will perhaps find twenty-five; and as each of these circles represents the work of one year, you will know that the tree is twenty-five years old. In spring, when the sap begins to move more briskly, it de posits everywhere between the wood and the bark, from the trunk to the farthest boughs of the tree, a uniform layer of a thick liquid, which moulds itself exactly upon the wood already formed. This layer stiffens during the year; it gets filled with the carbon left in it atom after atom, by each drop of the descending sap as it goes by, and thus insensibly becoming organised and hardened. When winter arrives to interrupt the work, it will have formed two ligneous, i. e. woody layers, as they are called. Of these, one belongs to the wood, and will never move again so long as the tree lasts, for it will be covered over, and as it were buried, by the successive layers yet to come; while, on the contrary, the other (layer) belongs to the bark, and is doomed to find itself perpetually forced outwards by the fresh layers, which will after a while insinuate themselves between it and the wood.

It is for this reason that the bark of old trunks of trees is so deeply furrowed, and that the dry scales may be picked off the surface without the slightest injury to the tree. It is part of the original bark, dead long ago. The old wood also is dead inside, and even when it is altogether gone, the glad youthful branches growing green in the sunshine will scarcely find it out! This accounts for those oaks which time has hollowed without destroying, as those of Allonville in Normandy, in which mass is said, and which is moreover the greenest tree in the country. But without going so far, who has not seen those hollow old willows, sometimes pierced with holes letting in daylight, yet proudly crowned

above by a forest of young boughs, as green and full of vigor as if the trunk were still in its prime? What was dead has departed, but all that has life in it remains, and that is enough for the tree.

Need I add that the descending sap, this steward of the vegetable, has also his workmen to supply with materials, as in our case, and that he is always falling in on his road with organs, all of which want different things from him? That here a flower has to be formed, there a fruit, there a leaf, or a bit of wood, and so on: and that a mysterious intelligence-the same that we have found everywhere else-presides over all these varied constructions, the materials for which are mixed together pell-mell, in the imperceptible thread of sap which oozes from the leaf to the bark? I recollect just as I am about to conclude, my dear child, that I once told you, you were a small temple in which God perpetually attests His presence, by a permanent miracle. You may now henceforth look upon a tree as something more than a bit of wood, yielding a pleasant shade. God is in it also.

CONCLUSION.

AND now, my dear little pupil, to what conclusion dc we come from all this? To that which I announced to you from the first. Throughout the length and breadth of creation, from the highest to the lowest grade, every living thing is subject to the same law. Everything eats, and eats nearly in the same manner, since everywhere the same substances furnish the feast. I laid down in my first letter that our feeding machine was reproduced even to the farthest limits of the animal kingdom, though always becoming more simple as the species descends in the scale. And afterwards, where we began the study of animals, I told you that in this machine lay the uniformity of their construction. Was I not right? and what could I add to all the proofs which have developed themselves one after another, to establish the fact of this uniformity of plan in the animal machine, in all its essential points? And it will be to the lasting renown of the illustrious Geoffroy St. Hilaire that it was, in the face of all the Academies and under the fire of very learned indignation, he proclaimed this truth, which one cannot lose sight of without losing one's way in a crowd of arbitrary fancies.

I return, then, to the definition which I gave you in speaking of the worm, and which is the final word of the idea I have been endeavoring to make you under stand. An animal is a digestive tube served by organs.