378 



NATURE 



[August 20, 1903 



gamia, " to quote a phrase of Prof. Sachs's, became the 

 true " Phaneroganiia, " their relative importance received 

 better recognition. In a recent classification — that of Prof. 

 Warming — out of twenty-three classes no less than eighteen 

 are assigned to Cryptogams. 



In spite of our vastly increased knowledge of the Crypto- 

 gamia, the flowering plants are still in the majority as 

 regards species. According to a recent census, out of about 

 175,000 known species of plants, about 100,000 or 4/7 are 

 phanerogamic. For our present purpose we may speak of 

 the flowering plants as the seed-bearing plants or Spermo- 

 phyta, for at least in recent vegetation the two characters, 

 the grouping of the reproductive leaves in a flower and the 

 formation of a seed, go together, and the latter is the 

 more definite and constant featpre. The Cryptogams, such 

 as ferns, mosses, seaweeds, and fungi, may, in contra- 

 distinction, be spoken of as the spore-bearing plants or 

 Sporophyta. In the vegetation, then, of the present day, 

 the seed-bearers are enormously predominant, not so much 

 in mere number of species as in importance, including, with 

 few exceptions, all plants of utility to man, and almost all 

 of conspicuous stature, and occupying vastly the greater 

 part of the earth's land surface. 



To what do the now dominant seed-plants owe their 

 success ? 



This is a diflRcuIt question, for all organisms are well 

 adapted or they could not exist, and nothing requires more 

 careful discrimination than the attempt to delermine the 

 exact factors which constitute the relative superiority of 

 one group over another in the struggle for life. Everything 

 depends on the conditions of the contest. 



In the simpler of the higher Cryptogams, such as ordinary 

 ferns, the spores are all of one kind, and on germination 

 give rise to an independent plantlet, the prothallus, on 

 which the sexual organs are borne. Fertilisation requires 

 the presence of water for the actively moving male cells, 

 the spermatozoids, to swim in. This condition may be 

 something of a handicap to the plant, but if water is pre- 

 sent, reproduction is fairly well ensured. In the more 

 advanced spore-plants, such as the Selaginellas, so 

 commonly grown in our greenhouses, the differentiation of 

 the sexes begins earlier, for the spores themselves are of 

 two kinds. There are numerous male spores of very small 

 size (microspores) and comparatively few female spores of 

 relatively large size (megaspores). In the group of the 

 water-ferns (Hydropterideae) only one of these large spores 

 is produced in each spore-sac, which then, if provided with 

 a special envelope, as in Azolla, may closely simulate a 

 seed. 



In the microspores, the prothallus is scarcely developed ; 

 the spore has practically nothing else to do but to produce 

 the spermatozoids. On the female side, provision has to 

 be. made for the nutrition of the embryo, and here there is 

 a comparatively bulky prothallus, though, as compared with 

 that of the ferns, it tends to lose the character of an inde- 

 pendent plant, and to become a mere storehouse of food- 

 materials. There are certain obvious advantages in this 

 heterosporous condition. The male spores are kept small 

 for easy dispersal, and can be produced in correspondingly 

 large numbers. The prothallial tissue is economised and 

 only formed where it is wanted, i.e. in connection with the 

 egg-cells from which the embryos arise. 



The differentiation of microspores and megaspores is, in 

 fact, comparable to that earlier differentiation of minute 

 active spermatozoids, and large stationary ovum, which 

 took place far back in the history. of both animals and 

 plants, and laid the foundation of sex. 



At the same time the heterosporous arrangement, as we 

 find it in Cryptogams, puts a new obstacle in the way of 

 the successful accomplishment of the act of fertilisation. 

 In order that this may happen.it is necessary that the two 

 kinds of spores should germinate together, as well as in 

 the presence of an adequate water supply. The necessary 

 association of the large and small spores' is, as a rule, left 

 to chance, the small spores being produced in enormous 

 numbers, so that the chance may be a good one. 



In the case of the great cryptogamic trees of the 

 Palaeozoic period the difticulty must have been a serious 

 one. We know that their spores often differed in mass in 

 the proportion of at least 100,000 to i, and when bodies of 

 such diverse weights were scattered by the wind from the 

 tops of lofty trees, the chances must have been enormously 



NO. 1764, VOL. 681 



against their coming to rest at the same spot. It was 

 perhaps to this difliculty that the series of adaptations lead- 

 ing up to seed-formation owed their first inception. 



If the microspores could be brought to the megaspores 

 while the latter were still attached to the parent plant, 

 much greater certainty of their union would be gained, for 

 adaptations would now become possible for catching the 

 small spores and retaining them in position. Some of the 

 Cryptogams now living have got as far as this ; the work 

 of an American lady, Miss Lyon, has shown that in some 

 species of Selaginella the microspores and megaspores meet 

 and the spermatozoids are discharged within the spor- 

 angium ; fertilisation is effected, and even an embryo may 

 develop before the megaspore is shed. In this last respect 

 these Selaginellas go beyond the seed-plants of the 

 Palaeozoic period, as we shall presently see. The first 

 advantage, then, to be secured was the occurrence of 

 fertilisation, or rather the bringing together of the two 

 kinds of spore, on the parent plant. This is one of the 

 constant characteristics of the seed-bearing plants ; the 

 process is spoken of as pollination,, for what we call the 

 pollen-grains are nothing but the microspores of the 

 Spermophyta. 



We will now see how the process actually goes on in 

 some of the simpler seed-plants of the present day. 



The seed-plants, as is well known, are divided into two 

 great classes, the Angiosperms, in which the seeds are 

 enclosed in a seed-vessel, and the Gymnosperms, in which 

 they are exposed. In the former, fertilisation is effected 

 by the growth of the pollen-tube through the tissues of the 

 young seed-vessel ; in the Gymnosperms the pollen falls 

 directly upon the young seed or ovule, and the pollen-tube 

 has only a short way to grow before reaching the egg-cell. 



The Angiosperms (Monocotyledons and Dicotyledons) in- 

 clude practically all our familiar flowering plants, but with 

 them we are not concerned at present. The question of 

 the origin of Angiosperms is one of the great unsolved 

 problems of botany, but it does not immediately touch our 

 present subject. It is to the simpler seed-plants — the 

 Gymnosperms — that we must turn for light on the origin 

 of the seed-plants as a whole. The Gymnosperms are 

 enormously the more ancient of the two classes, extending 

 back through the whole of the Carboniferous period into 

 the Devonian, while the Angiosperms, so far as we know, 

 only appeared quite late in the Mesozoic period. 



The most familiar of the Gymnosperms — the Coniferse or 

 cone-bearing trees — are themselves too far advanced on the 

 seed-bearing line for our purpose. We will concentrate our 

 attention on a family which, of all living flowering plants, 

 stands nearest to the Cryptogams, namely, the Cycads. 

 This group, not very well known to the non-botanist, but 

 of which a splendid collection will be found in the palm- 

 house at Kew, is now a small one, including nine genera 

 and about seventy species, distributed through the tropical 

 and sub-tropical regions of both the old and new worlds. In 

 habit these plants, which may rise to the stature of small 

 trees, bear some superficial resemblance to palms ; the 

 agreement with ferns is, however, much more striking. 



In the genus Stangeria from tropical Africa, the leaves 

 bear so close a resemblarfce to those of some ferns in form 

 and veining that the plant, before its fructification was 

 known, was described by competent botanists as a species 

 of the fern-genus Lomaria. 



In all Cycads the male fructifications are in the form of 

 cones ; the pollen-sacs are borne in great numbers on the 

 under surface of the scales of the cone. In all the genera 

 but one, the female fructifications are also cones, each scale 

 bearing two large ovules. In the type genus Cycas, how- 

 ever, there is no specialised female cone at all. The fertile 

 leaves are borne in rosettes on the main stem, alternating 

 with zones of the ordinary vegetative leaves. 



The fertile leaves are of large size and compound form, 

 and usually each of them bears several ovules, which, 

 whether fertilised or not, grow to a great size, sometimes 

 as big as an egg-plum. They are in some species of a 

 bright red colour, and contrasting with the yellow woolly 

 leaves on which they are borne, are conspicuous and 

 beautiful objects. 



In thus bearing its seeds on leaves so little modified, and 

 springing, like the ordinary leaves from the main stem, 

 Cycas is the most fern-like genus of flowering plants. 



The ovule, at the time when pollination takes place, is 



