INTRODUCTION.
That the substance of the more perfect plants consists of layers of different constitution was a fact that could not escape the most untutored observation in primitive times; ancient languages had still words to designate the most obvious anatomical components of plants, rind, wood and pith. It was also easy to perceive that the pith consists of an apparently homogeneous succulent mass, the wood of a fibrous substance, while the rind of woody plants is composed partly of membranous layers, partly of fibrous and pith-like tissue. The obtaining of threads for spinning from the rind of the flax-plant, for instance, must have suggested some idea, if only a vague one, in the earliest ages of the way, in which the fibrous could be separated from the pulpy part of the bark by decay and mechanical treatment. Neither Aristotle nor Theophrastus failed to compare these components of vegetable substance with corresponding ones in animal bodies, and it has been already shown in the first book how Cesalpino, following his masters, took the pith for the truly living part of the plant and the seat of the vegetable soul, and applied this idea in his morphology and physiology. He remarked that the root generally has no pith, and that the part of the root which answers to the wood of the stem is often soft and fleshy; the composition of the leaves from a green and succulent substance and strands of fibres at once suggested a certain resemblance to the green rind of the stem; and it was evidently this which led him to consider that not only the leaves, but also the leaf-forms of the flower-envelopes had their origin in the rind of the
