Plants are divided into two great groups, the _Cryptogams_ or Flowerless Plants and the _Phanerogams_ or Flowering Plants. In the former the reproductive organs are not true seeds containing an embryo of the future plant, but mere cells or _spores_, which give rise directly to a thread or mass of threads, to a cellular membrane, or to a cellular body of more or less complexity of form from which the flowerless plant is afterwards developed; while in the latter the reproductive organs are flowers that give rise to true seeds, each of which contains the embryo plant. The _Cryptogams_ are subdivided into four groups--the _Thallophytes_, the _Charales_, the _Muscineæ_, and the _Vascular Cryptogams_. The first of these includes all the very low forms of vegetable life, the simplest of which (_Protophyta_) are minute plants, each consisting of a single microscopic cell that multiplies by a process of budding, no sexual organs of any kind being produced. Some of these minute unicellular organisms contain chlorophyll--the green colouring matter of plants, by the action of which, under the influence of light, the plant is enabled to decompose the carbonic acid gas of the atmosphere, using the carbon for the purpose of building up its own substance, and setting free the oxygen into the air again. Others contain no chlorophyll; and these, having no power of feeding on carbonic acid gas, are more or less dependent on organic matter for their supplies of carbon. Only very slightly removed from these minute plants are the _Algæ_ of fresh and salt water, varying in size from microscopic dimensions to enormous plants, the lengths of which may reach many yards and the weight several stone. They contain chlorophyll, and can therefore avail themselves of inorganic food material; and although some multiply only by repeated subdivision of their cells, others develop sexual organs by the union of which fertilised spores are formed. The nature of these Algæ will be more fully described presently; and we will go no further now than to justify the location of such large and conspicuous plants (as many are) so low in the scale of vegetable life by stating that they are entirely cellular in structure, never producing true vessels such as we see in higher plants; and that though some of them develop parts which more or less resemble the leaves and roots of higher forms, the former are far more simple in structure and function than true leaves and the latter are never engaged in the absorption of food from the soil to which they are fixed.