The primary groups into which flowering plants are divided, and in which therefore the families or orders are themselves comprised in the classification at present accepted, being founded upon the degree of cohesion and adhesion in the petals and stamens, are undoubtedly somewhat artificial. The problem of how the orders are themselves to be combined into natural groups is one which still engages the attention of systematic botanists. The vegetable kingdom is divided by Dr. Lindley into seven classes: FLOWERLESS PLANTS (CRYPTOGAMS) { A Thallus is a fusion of root, { stem, and leaves into one general { mass, and Thallogens are { Stems and leaves { destitute of breathing pores, I. THALLOGENS { imperceptible. { and multiply by the formation { of spores, in their interior or { upon their surface. { Beyond Thallogens are multitudes { of species, flowerless { like them, but approximating { to more complex structures, { sometimes acquiring the stature { of lofty trees with breathing { Stems and leaves { pores; their leaves and stems II. ACROGENS { quite perceptible. { distinctly separated; they multiply { by reproductive spores { like the Thallogens. Their { stem, however, does not increase { in diameter, but at their { summit, as the name of the { class indicates. FLOWERING PLANTS (PHANEROGAMS)