126 



KNOWLEDGE 



[June 1, 1894. 



latter occur as early as the Devonian period, but the 

 prevailing type of vegetation down to the close of 

 Palipozoic time continued to be cryptogamic. Angio- 

 sperms possibly existed as far back as Permian times, but 

 it is only in the chalk that their remains begin to be 

 abundant ; the vast niajority of Mesozoic plants seem to 

 have belonged to the gymnospermous type. Plants with 

 conspicuous dowers only date from Tertiary times ; they 

 increase in number and importance as we approach the 

 present day. 



Although the plants entombed Ln the rocks are only an 

 inconsiderable fraction of the numbers that formerly 

 existed, the general succession just indicated is fuUy made 

 out, and as the palieontological evidence accumulates it 

 tends more and more to establish the view that coloured 

 blossoms are, geologically speaking, of comparatively recent 

 origin. The vegetation of the earlier geological epochs 

 was marked by a singular miiformity of character ; not 

 only were there fewer species than now, and these widely 

 distributed over the globe, but the monotonous green of 

 PalsBOzoic and Mesozoic forests was unrelieved by gay 

 blossoms such as adorn our fields and orchards. We are 

 indebted to geology for another important fact ; fossil 

 plants occur which have no near relatives in the existing 

 flora. Intermediate forms which cannot properly be 

 classified with any linng family are met with ; in others 

 the characters of several modern groups are blended. 

 Although these generalized forms rather upset our systems 

 of classification, they have an important bearing on the 

 origin of li^dng plants. But what a different aspect, when 

 the coal plants were gi-owing in primeval luxuriance, the 

 landscape must have worn fi'om that on which we are 

 accustomed to look ! Odd, uncouth lepidodendra of 

 arborescent growth, huge reed-like calamites, gigantic 

 ferns stretched in interminable forests, clothed in one 

 im varied tint of sombre green. How different is the scene 

 which Nature now presents ! — mountains glowing with the 

 purple bloom of heather ; hillsides where the furze has 

 spread its cloth of gold ; meadows bright with daisies, 

 rauimculi, and cuckoo-flowers ; banks where the wild 

 thyme and bluebell grow ! The contrast affords a hint of 

 the transformation in our world effected by the introduction 

 of flowers. 



Our knowledge may not enable us to describe all the 

 minute steps which led to this remarkable change, but we 

 can at least mdicate with great probability the nature of 

 the process and some of the agencies which contributed to 

 bring about this result. To suppose that each species of 

 plant was independently created as we now see it, implies 

 not one creation merely but many successive creations ; 

 moreover, it leaves unexplained all the curious affinities 

 which exist among the members of the vegetable kingdom. 

 The gradations of structure, the geological succession and 

 the peculiarities of plant growth are much more intelligible 

 when we view the plants which now inhabit the earth as 

 the lineal descendants of those which lived during the 

 earlier ages of geology. From the nature of the case, the 

 theory of development does not admit of actual demon- 

 stration ; still the evidence in support of it is such that 

 its advocates are entitled to claim a verdict on the mass of 

 indirect and circumstantial evidence. 



Among palaeozoic cryptogams we have evidence of the 

 existence of structures which, with comparatively little 

 modification, might be converted into what we now regard 

 as flowers. The abundant remains of lepidodendra Ln the 

 coal measures testify to the important place attained by 

 the group of lycopods, or club mosses, in the Palseozoic 

 flora. To this family might very well have belonged the 

 archetype from which our modern blossom-bearing plants 



have come. Our knowledge of this group is derived both 

 from fossil remains and from forms still extant. The 

 selaginellas, so commonly cultivated in greenhouses, are 

 examples; also the little club moss, Lijcopodiumselaiiitioidfs, 

 of otu' highland moors. The last mentioned, though a 

 diminutive form, possesses special interest, being one of 

 the few vascular cryptogams native to this country which 

 produce two kinds of spores. This heterosporous character 

 was, however, a common feature of extinct lycopods ; both 

 large and small spores have been detected in great numbers 

 in coal. 



The internal anatomy of the lycopodiacese is somewhat 

 complex, but their external organization is simple. A 

 club moss consists of a cylindrical stem covered with 

 overlapping leaves, spirally arranged, of small size rela- 

 tively to the stem, and always simple or undivided. The 

 stem branches in a peculiar forked manner, which gives 

 the plant its characteristic candelabra-like form. Existing 

 lycopods are creeping plants, seldom exceeding two feet in 

 height, but many extinct species attained the dimensions 

 of large trees. On the ends of certain branches the 

 leaves are crowded together, giving the terminal portion 

 of each shoot some resemblance to a pine-cone. The 

 crowded leaves on this portion bear, on their upper 

 surfaces, little sacs called sporangia. Certain of these 

 sacs contain very numerous small rounded bodies, the 

 microspores ; others have fewer spores of larger size, dis- 

 tinguished as macrospores. Sacs containing the small 

 male spores are termed microsporangia ; those having the 

 large female spores, macrosporaugia. ^Yhen ripe, a spor- 

 angium bursts and discharges its spores, which arc 

 scattered by the wind. Should a spore alight on a favour- 

 able spot, it germinates after a time and gives rise to a 

 structure called a prothaUus, which is really an independent 

 plant. This stage in the life-history of a cryptogam is, 

 however, much better seen in ferns, where the prothaUus 

 is extirely expelled from the spore and attains a higher 

 degree of independent development. The prothaUus 

 throws out root-hairs, nourishes itself and grows, but the 

 leaf-like form it assumes bears not the remotest resem- 

 blance to the parent fern from which it sprang. This 

 phenomenon, characteristic of the higher cryptogams, is 

 known as the "alternation of generations." Similar phases 

 are observed in certain animals, the medusa or jelly fishes 

 for example. In the course of its development a fern 

 passes through two distinct phases ; first, the spore-bearing 

 stage or sporophyte, repi-esented by the fern frond ; 

 secondly, the egg-bearing stage, the oophyte or prothaUus. 

 As we ascend in the scale of vegetable life, the egg-bearing 

 or sexual generation diminishes in importance, while the 

 sporophyte preponderates more and more. In club 

 mosses the prothaUus has all but lost its independence ; in 

 the case of selaginella it is formed almost entirely within 

 the spore, only a small part being extruded when the 

 spore ruptures. Some of the lycopods are isosporous — that 

 is, they have, like the ferns, but one kind of spore. "Where 

 this is the case, the prothaUus developed fi-om the spore 

 bears two sets of sexual organs ; the prothaUus of one of 

 the heterosporous cryptogams, on the other hand, produces 

 sexual organs of one kind only. Antheridia appear on the 

 prothaUus developed from a smaU spore ; archegonia on 

 that fi-om a large one. The former are the male organs, 

 and from them are emitted numerous antherozoids, 

 minute cUiated bodies, which swarm over damp surfaces 

 in aU du'ectious. The archegonia are microscopic flasks, 

 each containing an egg-cell or oosphere ; they are entered 

 by one or more of the locomotive antherozoids, which 

 coalesce with the egg-cell ; the latter is thereby fertUized, 

 and soon grows by cell division into a plant resembling 



