June 1, 1894.] 



KNOWLEDGE 



127 



that from which the spores were originally obtained. The 

 life-history of a vascular cryptogam is, so to speak, a story 

 completed in two volumes. 



Microscopic research has revealed a most interesting 

 relationship between flowering plants and the heterosporous 

 cryptogams. When the development of the pollen grain 

 in the anther of an ordinary flower is studied and com- 

 pared with that of a microspore, the two are foimd to agree 

 in a remarkable manner. The sporangium corresponds in 

 all essential points with the pollen-sac, and its generating 

 tissue develops in similar fashion to that from which the 

 pollen grains originate. In both cases an archesporium 

 is produced by the division of a hypodermal cell ; this 

 tissue next divides into a tapetal layer and a row of 

 mother-cells ; the tapetal layer dissolves, isolating the 

 mother-cells, each of which then forms in its interior four 

 daughter-cells, which are the spores or pollen grains as 

 the case may be. Not only are the antecedents of micro- 

 spores and pollen grains alike, but their subsequent 

 histories offer many points of resemblance. Pollen grains 

 are known in numerous instances to form in their interior 

 one or more vegetative cells, which can hardly be regarded 

 as other than a rudimentary male prothallus, such as is 

 commonly developed by a microspore. 



There is another bond of connection between flowering 

 and flowerless plants of equal or even greater importance. 

 In the interior of the ovule, or yoimg seed, both of angio- 

 sperms and gymnosperms a special cell is developed, called 

 the embryo-sac. When the history of this cell is traced 

 back its development is found to be exactly that of a spore. 

 Certain structures are also formed in its interior bearing 

 the closest analogy to the internal prothallus observed in 

 the macrospore of selaginella. These are most obvious in 

 the embryo-sacs of gymnosperms, where the prothallus is 

 represented by the endosperm, while the corpuscula, or 

 secondary embryo-sacs, arising on this are the undoubted 

 equivalents of the archegonia of ferns and other cryp- 

 togams. The gymnosperms thus stand midway between 

 vascular cryptogams and angiosperms ; but even within 

 the embryo-sac of the latter, in the so-called antipodal 

 cells, may still be detected vestiges of the oophyte or 

 sexual generation, that structure so characteristic of the 

 flowerless class. 



An alternation of generations can thus be traced through- 

 out the greater part of the vegetable kingdom, from the 

 lowest scale mosses through the urn mosses, ferns, horse- 

 tails, lycopods, and conifers, up to the highest members 

 of the phanerogamic division. But of more importance 

 for our present purpose is the certain identification of the 

 pollen grain and embryo-sac of flowering plants with the 

 microspore and macrospore of the older cryptogams. The 

 stamen of a flower turns out to be simply a peculiar form 

 of microsporangium, while the ovule is a macrosporangium, 

 containing but one macrospore, or occasionally developing 

 several. It follows, therefore, that we have only to enlarge 

 our conception sufficiently to see in the spore-bearing cones 

 of the lycopods structures of essentially the same nature 

 as flowers. All the materials that go to the making of a 

 flower could thus have been furnished by the flowerless 

 flora of Paleozoic ages. 



An important change, which marked the transition from 

 cryptogams to flowering plants, must now be mentioned, 

 and to this the animal kingdom furnishes a striking 

 analogy. The lowest vertebrates, such as fishes, are 

 oviparous ; the ova are discharged and afterwards incu- 

 bated. Mammals, on the other hand, are viviparous ; 

 the young are hatched within the body of the parent. 

 The young of the kangaroo and other marsupials, which 

 constitute the lowest order of mammals, are still very 



immature at birth. Analogous conditions are found among 

 plants. Cryptogams are all oviparous ; the macrospore, 

 which may be regarded as the ovum or egg, separates from 

 the parent plant before fertilization. Phanerogams, on 

 the other hand, may be described as viviparous, since they 

 retain the macrospore or ovum until it has developed an 

 embryo. The presence of an embryo constitutes the 

 distinction between a seed and a spore. Unless an embryo 

 be present a seed cannot germinate, since germination is 

 simply the emergence of the embryo from the coats of the 

 seed. An extreme case of this retention is seen in the 

 mangrove, where the seed germinates while still attached 

 to the tree ; the embryo sends down its long radicle into 

 the mud, and only quits its hold of the parent when it has 

 become firmly, established. Orchids and many parasitic 

 plants have seeds with exceedingly minute and imperfect 

 embryos, recalling the undeveloped oft'spring of the 

 marsupials. 



The retention of the egg is attended with a manifest 

 advantage ; plainly the viviparous method of reproduction, 

 which obtains in the higher divisions of the two organic 

 kingdoms, is much more economical than the other. By 

 the change to the viviparous condition several structures 

 present in the cryptogams are rendered useless, and a 

 disused organ invariably degenerates ; the prothallus and 

 its adjuncts, having no longer any function to perform, 

 must inevitably begin to atrophy. The rudimentary 

 structures appearing in the embryo-sac of phanerogams 

 can in this way be accounted for. The life-history of a 

 cryptogam extends, as we have seen, to two volumes ; it 

 now appears that the life-history of a phanerogam is a 

 second edition of the same story, somewhat abridged and 

 completed in a single volume. 



The life-history of certain ferns occasionally undergoes a 

 corresponding abbreviation. In the phenomena of apospory 

 and apogamy we have departures from the ordinary course 

 of development, closely akin to what would be required 

 for the conversion of a cryptogam into a phanerogam. 

 Apospory occurs when the production of spores is omitted, 

 the prothallus growing immediately on the fern frond ; 

 apogamy, when the female organs are not developed, and 

 the fi'ond is formed by vegetative growth directly from the 

 prothallus. 



There is another fact of which account must be taken. 

 In difl'erent groups of plants, in proportion to the com- 

 plexity of their organization, the female cell tends to 

 increase in size and importance. This is probably accom- 

 panied by a chemical or physiological enrichment of the 

 substance of the egg-ceU, rendering a higher degree of 

 protection desirable. The enclosure of the embryo-sac 

 within the ovule becomes in these circumstances an ad- 

 vantage. But by this investment, and by the ovule 

 remaining attached to the parent plant, the microspore is 

 of necessity reduced to the condition of a parasite, and the 

 conversion of the male prothallus into a pollen tube 

 becomes intelligible as a case of degeneration. 



The closed seed-vessel of angiosperms, there can be little 

 doubt, has in like manner been acquired for the purpose of 

 excluding fungus spores, bacteria, and other destructive 

 germs from the ovules. Van Tieghem found that when 

 the pistil of a flower was opened the ovules could not be 

 directly fertilized, but were invariably attacked by bacteria. 

 The resinous secretions of conifers act as a germicide, 

 rendering less essential the protection of the seeds, which 

 is the role of the pistil in angiosperms. 



The gradations between stamens, petals, and sepals seen 

 in the water lily, and the conversion of stamens into petals 

 in the garden rose, suggest a possible variation which 

 would explain the first appearance of the floral envelopes. 



