HOW THE PLANTS DEVELOPED THEIR ORGANS

BEFORE proceeding to give an outline of the main groups of extinct plants which composed the Upper Palaeozoic flora, it may be useful to sketch* briefly some of the lines along which the higher land-plants became differentiated, with regard to the types of structure that they show in their various organs. From the simplest unicellular plants one passes to multicellular forms with an undifferentiated plant-body known as a thallns. The seaweeds are typical thallus-plants; except in the highest forms they show little differentiation into root, stem, or leaves, and have practically no internal conducting system. The simplest land-plants, having escaped from the watery medium, must develop a root system for attachment and for water absorption; a conducting system; strengthening tissues, since they are no longer held up by the surrounding water; and breathing pores.

Some of the early Devonian plant fossils have not got very far in the differentiation either of organs or of tissues. The underground portion of Rhynia, for example, has the same structure as the aerial stem, and differs only in bearing root-hairs, while Asteroxylon has not even got these. In later Palaeozoic times, however, we find numerous types of highly elaborate root-systems, often differing so much structurally from the stems they bore that they have been mistaken for independent plants until found in connection.

THE LAND-PLANT’S GREATEST LEGACY: THE STEM SYSTEM WE have already remarked that the central strand of the primitive land-plant served the double purpose of support and water-conduction, and this dual function has been retained throughout the whole subsequent history of every group of the higher plants. Increase in size, however, together with branching and the development of leaves each supplied with its own strand led to complex developments in the conducting system, which followed different lines in the various groups of plants. The primitive strand (or stele) of Rhym’a, consisting of a solid cylinder of wood surrounded by a sheath of bast (phloem), became more complicated in Asteroxylon, which itself may be regarded as a basic form from which the type of stele found in the club-mosses and their allies may have been derived. Along some lines of development, the woody projections of the stele may branch in a complicated manner; along others, a central pith may develop in the wood, and the cylinder thus formed may be broken up in various ways by branches and leaf traces.

With growth in size, a simple woody strand becomes insufficient, and new wood must be formed, either in small patches, or in layers all round the stele. When this secondary wood continues to be formed from a growing layer, the herbaceous plant becomes a tree. So far as the primary wood of the stem is concerned, two main types of growth have been observed : an older type, the centripetal, developing towards the centre of the stem, and a newer, the centrifugal, developing from within outwards. The latter gradually replaces the former in various groups of plants; the presence of centripetal wood in the stem is recognised as an archaic character, and is only found among living plants in the cycads, themselves the relics of an ancient group.

One other anatomical character of many Palaeozoic plants may be briefly mentioned; the frequent occurrence of bands of strengthening tissue in the outer part of the stem. This no doubt balanced a certain weakness in the central woody column in cases where the latter was mainly concerned with water-conduction. Plants in those days developed varied mechanisms in order to adapt themselves to the conditions under which they lived, just as do the plants of the present, and there is nothing in the structure of any known fossil

plants which even suggests that they lived under conditions which cannot be paralleled at the present day.

THE LEAF MAKES ITS APPEARANCE

AMONG the early land-plants, in addition to the leafless Rhyuia, there are two different types of leafy appendage— first the little scaly outgrowths which clothed the stems of plants like Asteroxylon, and which may have given rise to the small crowded leaves of the kind found in the club-moss group, and secondly, the flattened branch-systems of some other plants which seem as if they were the precursors of the fern type of compound frond.

The green leaf is the special organ concerned with carbon assimilation, which in the simplest plants may be performed by the whole vegetative body, and there is no reason why leaves may not have originated in several different ways. Among living flowering plants, for example, the apparent leaves of the ‘butcher’s broom ‘are (botanically speaking) flattened stems. Thus even when a part of the plant has been specialised for its particular function, it may also take on other duties.

The variety of leaf-form and venation is almost as great among Coal Measure plants as among flowering plants of to-day. There are the linear or lanceolate whorled leaves of the tree horse-tails or Cala?nites, graceful and feathery; the conifer-like foliage of the giant club-mosses; the wedge-shaped and fringed leaves of the Sphenophylls; the huge strap-like parallel-veined leaves of Cordaites; and among the seed-ferns almost every type of simple and compound frond, including some which might easily be mistaken for the leaves of modern flowering plants, as well as many which at first sight are almost indistinguishable from those of living ferns. As A. C. Seward has said :

‘In the course of ages, similar results have been achieved by many diverse groups of plants; age after age there has been a repetition of unconscious effort towards the same end. Groups which had reached what might be regarded as an advanced state of efficiency became extinct; after a long interval new creations repeated with little or no change in plan the structural design produced by long-forgotten and possibly blindly ending lines of evolution.’

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