166 



DISCOVERY 



The Origin of the Vegeta- 

 tion of the Land 



By A. C. Seward, F.R.S. 



Matter of Downing Collegt and Pro/essor o/ Botany, Cambridge 



Even though we shall never solve many of the problems 

 connected with the early history of the earth, there 

 will always be a fascination in getting the most out 

 of such facts as are available, and drawing upon our 

 imagination for the rest. The origin of life is one of 

 •the oldest and most attractive problems that will 

 long continue to exercise speculative minds. A recent 

 contribution to this subject has recently been made 

 by Dr. Church,' of Oxford, which deserves careful 

 attention ; but it is not with this that we are now 

 primarily concerned. The same author has dealt in 

 a masterly manner with the origin of the vegetation 

 of the land." The hj^sothesis he elaborates is not 

 only ingenious, but the arguments are logically pre- 

 sented, though in a style which, it must be confessed, 

 is not easy reading even for the specialist. The problem, 

 briefly, is this : W hat was the origin of the vegetation 

 of the land as distinct from that of the water ? 



The majority of plants live on land : trees, shrubs, 

 herbaceous flowering plants. Ferns, and Mosses, are 

 either wholly or with comparati^'ely few exceptions 

 dwellers on land. They draw their supply of carbon 

 from carbon dioxide, a gas which forms one of the 

 constituents of the atmosphere ; the atmosphere 

 contains about three parts of carbon dioxide in 10,000. 

 The structure of the bodies of the more highly organised 

 plants is intimately connected with the needs of a hfe 

 ■on land. The roots take up raw material in solution 

 from the soil ; they act as absorbing organs as well as 

 holdfasts ; the stem and branches bear the leaves 

 and flowers, or other reproductive organs ; some of 

 their tissues are concerned with the conduction of 

 water and with the transport of the organic compounds 

 manufactured in the green leaves to those organs 

 where building-material for new cells is needed. A 

 land plant absorbs water only from the ground, and 

 it must, therefore, be provided with an efficient system 

 of channels for conducting water to all parts of the 

 plant-body. In the higher plants the method of 

 reproduction also is in harmony with land conditions. 

 Thus, some of the essential features of a typical land- 

 plant— for example, a Pine-tree or an Oak— are : 

 (i) that it draws from the soil and the air all the raw ma- 

 terial from which it builds up its substance ; (ii) that 



' The Building oj an Autotrophic Flagellate. Oxford 

 University Press, 191 9. 



• Thalassiophyta and the Subaerial Transmigration. Oxford 

 University Press, 191 9. 



it is able, by means of certain specially strong tissues, 

 to maintain an erect position and to resist the force 

 of the wind ; (iii) that it can reproduce its kind 

 without the aid of any reproductive cells endowed 

 with the power of locomotion. A water-plant, on the 

 other hand, is supported by the medium in which it 

 lives. In a seaweed, moreover, which is entirely 

 surrounded by water, absorption is not localised in a 

 special organ, but an efficient water-supply is assured 

 by the permeability of the whole plant, and no special 

 conducting tissue is needed. 



In the Ferns, Mosses, and Algse ' the act of fertilisa- 

 tion is accomplished, with rare exceptions, by means 

 of free-swimming or floating male cells, and motile 

 or non-motile female cells (eggs). This character is a 

 legacy from ancestors which passed their lives in water, 

 but the higher plants have lost this relic of an aquatic 

 ancestry. Ferns and Mosses, for example (in contrast 

 to aquatic Alga, which are fully equipped for hfe in 

 water), may be described as amphibious ; though they 

 Uve for the most part on land and produce spores 

 dispersed by the wind, they are still dependent for 

 fertilisation upon water, as their male reproductive 

 cells swim to the female cells. 



We know, from a study of the rocks, that the present 

 vegetation of the land represents merely a stage in a 

 succession of plant-associations which have colonised 

 the earth during millions of years ; and we are also 

 in a position to assert that no satisfactory evidence 

 has so far been discovered of the existence of a land- 

 vegetation before a certain period in the history of 

 the earth. 



At what stage, then, of geological histor>' did plants 

 first establish themsel\-es on the land ? The oldest- 

 known rocks are spoken of by geologists as Archaean : 

 these rocks form a considerable part of the North- West 

 Highlands of Scotland and occur in North and South 

 Wales, in the Malvern Hills, and elsewhere. They 

 consist of granites and other coarsely crystalline rocks 

 of igneous * origin, covered in some regions by a great 

 thickness of grit and sandstones formed by the action 

 of denuding agents, rain and frost, and either deposited 

 as sheets of pebble-beds, grit, and sand on the surface 

 of the land, or accumulated as sediments under water. 

 The Archa;an rocks reach an enormous thickness, and 

 afford unmistakable evidence that their formation 

 occupied a length of time in the early history of the 

 world which is beyond our comprehension. They 

 give very little information about the kind of life that 

 existed when thej' were being formed, and there is at 



' A large class of plants, most of which live in water ; it 

 includes seaweeds, also many simpler types of pljints living in 

 fresh-water and on damp surfaces. 



* Formed by volcanic agency, or from molten material crys- 

 tallised under great pressure and at a considerable depth 

 below the surface. 



