The acrogens take an immense range in the scale of organisation, from the ferns (which appear but little inferior to the exogenæ or endogenæ, have stems and leaves, and in some cases, as in hot and moist climates, assume the size and form of a tree), to the very lowest state of vegetable existence, consisting of simple cells uniting into strings or forming simple threads, such as the green algæ which form on stagnant waters and damp ground or wood-work, and the mould or mildew which forms on all decomposing substances. The general name for these acrogens, "cryptogamia," which has been in use for a long time and is commonly still used, indicates that the reproductive organs are invisible, hence the expression used by one of Shakespeare's characters, "We have the secret of fern seed, we walk invisible;" but this is not really the case, for the "sorri" at the back of fern-leaves, are vessels filled with spore cases each having a number of angular seeds within it; the lower tribes of the acrogens do not commonly grow from seed but by an extension of their several parts by the development of the cells of which they are composed, and by their separation into portions. Few of this tribe have anything like true woody texture, except their highest order, the ferns, which form some of the most beautiful objects in the vegetable world. Few of the family of acrogens are of much direct use to man, the mushroom tribes are very generally eaten where they abound, the lichens of the arctic regions form the food of the reindeer (the greatest friend of man in these cold climes) as well as, in part, the food of man himself; but although these lowly plants serve man but little, directly, there is not a shadow of doubt that they have as important an office to fulfil as any other family or tribe of organised creatures, whose purpose may meet the eye more plainly. For all the members of creation form, as it were, the links of one great chain, and were but one removed, though it might perchance be only some poor weed or lowly moss, yet might it cause the whole to be annihilated; for certain earthy matters enter into the structure of all plants, and it appears to be the wonderful office of some of these lowest tribes of plants to prepare this earthy matter for its reception into the systems of higher organisms, for as silica is one of the primitive rocks of the earth and is only found in fragments, from the largest to the sand on the sea-shore, which is nothing but a collection of minute fragments of quartz worn small by attrition, yet a grain of sand is a gigantic mass of rock in comparison with the thin porous hollow shells of the Diatomaceæ, &c., and by far too large to be absorbed or dissolved so as to be taken into the systems of other plants that may require it, which plants would cease to exist if this earth were not thus prepared for them; now these are the corn-bearing plants, the most useful to the animal world, and upon which it in