74 



NA TURE 



[_May 26, 1 88 1 



tardily become transformed into " Amblystomes," their 

 morphologic differentiation being unchecked by the act of 

 reproduction. Similarly among primitive terrestrial Algas, 

 those in which sexuality was deferred until late had a 

 longer period of purely vegetative life, and therefore not 

 only felt more strongly the influence of new conditions, 

 but had a longer time in which to adapt themselves 

 and thus become diversified in type. The resultants of 

 this elaboration are represented now by the Mosses and 

 Hepaticse. In the Mosses the spore gives birth to a con- 

 fervoid thallus called "protonema,"a reversion apparently 

 to the primitive ancestral Alga. This elementary thallus, 

 not being arrested by the appearance of sexual organs, is 

 susceptible to subsequent differentiation ; foliary buds are 

 given off in places from its ramifications, the multiplica- 

 tion of cells at these points becomes regular, and little by 

 little small lamince assume the form of leaflets on a stem 

 supported by radicles. These radicles are capable of 

 producing new plants, and mosses propagate so energeti- 

 cally that extensive carpets may be formed without the 

 aid of reproductive organs, and in some species fruiting 

 plants are rarely met with. This great vegetative power 

 seems entirely due to the absence or rarity of sexual 

 function. The reproductive organs when present are 

 however of the greatest morphological importance. These 

 are distinguished as "antheridia," or male, and " arche- 

 gone," or female. At maturity antherozoids escape 

 from the antheridia and impregnate the archegone. The 

 " oospore " contained in the archegone produces a new 

 cellular plant, which develops more or less within the 

 archegone in which it is born, and finally becomes the 

 organ called " fruit " in the Mosses. This so-called fruit 

 is in reality a distinct plantlet, called a " sporogone," which 

 by asexual generation or simple multiplication gives birth 

 to the spores, and these spores, falling in damp places, 

 again give rise to new thalli or moss plants. This 

 alternate generation is unknown among AlgJB. We 

 have thus in the Mosses a new point of departure, the 

 more important generation, being analogous to Algas and 

 tardily sexual, take on very complete morphological cha- 

 racters ; the other generation, agamous, subordinate, and 

 incapable of disengagement from the archegone in which 

 it is formed, yet fundamentally an independent plant. 

 The Hepaticre are similar in growth, and both together 

 present a stationary group which have elaborated a special 

 kind of organic differentiation, but in a direction limited 

 by biologic conditions. Derived from cellular thalli with 

 " tardy sexuality," evolution has acted exclusively on the 

 first generation ; while the second, of newer origin and 

 free from heredity, would have been susceptible of far 

 more complete differentiation. The truth of this hypo- 

 thesis becomes apparent when Ferns, Equisetaceas, and 

 Ophioglossea; are studied. 



The origin of these three groups is similar to that of 

 the Mosses and Hepaticse. Their spores give birth to a 

 cellular thallus or " prothallus," which "precociously" 

 produces numerous archegones and antheridia. The 

 same process takes place as in the case of Mosses, e.xcept 

 that the resulting "sporogone" is vigorous and speedily 

 effaces the ephemeral life of the sexual plant. It promptly 

 frees itself and takes root, its tissues become e.xtremely 

 diversified, and fibres and vessels, histological elements 

 previously unknown, appear, and plants known as ferns, 



horsetails, &c., result. On the leaves of this highly- 

 developed sporogone the sporangia are born which 

 produce the spores, whose germination gives birth to the 

 sexual prothallus. The precocious and abundant deve- 

 lopment of sexual organs almost immediately arrests the 

 differentiation of the prothallus, and the primordial aerial 

 Alga becomes completely subordinate. On the other 

 hand the sporogone which succeeds became more and 

 more developed and commenced a series which step by 

 step has led finally to the most highly organised and most 

 recent group of plants, the Angiosperms. The evolution 

 which has given us those plants, which seem to an inat- 

 tentive observer to form nearly the entire vegetation of 

 the earth, is in the authors' opinion the result of a circum- 

 stance, doubtless almost insignificant in its commence- 

 ment, and of which the first eftects were to arrest by a 

 precocious sexuality the organic differentiation of some 

 of the primordial terrestrial plants. While everything 

 seemed to unite to favour the evolution of those types with 

 permanent thalli, and which produced Mosses and the 

 Hepaticas, other thalli of lower development found in the 

 very causes which limited their differentiation, the starting 

 point of a new vegetative system, that of the sporogone, 

 the preponderance of which soon became manifest. In 

 the Rhizocarps we see this species of development in a 

 more advanced stage than in the Ferns. The sporogone 

 has become more and more preponderating, and the pro- 

 thallus scarcely disengages itself from the envelopes of 

 the spore. 



Ferns occasionally exhibit a tendency to a separation 

 of the sexes, for the prothallus may be either male or 

 female, but in the Rhizocarps dicecity is more nearly 

 realised, for the spores themselves are of two sexes — 

 microspores and macrospores. The germination of the 

 microspores consists simply in the production of tubes 

 scarcely divided into cells, in one of which the anther- 

 ozoids are produced. In the macrospore, though a rudi- 

 mentary prothallus is at first more or less apparent, this 

 is quickly concealed by the extension of the sporogone 

 developed within one of the archegones. With the dis- 

 appearance of the rudimentary prothallus almost the last 

 trace of the primordial cellular Alga disappears. 



The prothallus is thus seen to be so reduced in the 

 Rhizocarps that it seems almost as if the sporogone were 

 disengaged directly from the macrospore. This sporogone 

 follows otherwise the same histological development as 

 in Ferns, but gives birth morphologically to a further 

 departure. Certain fronds become differentiated into 

 "sporocarps," a kind of fruit comprising both micro- and 

 macrosporangia, and which in Maisilia attain remarkable 

 complexity. This is the highest point of evolution seen 

 in existing Cryptogams, for the Lycopods are rather a 

 parallel development than an actual advance beyond the 

 Rhizocarps. They are divided into Isospores, or true 

 Lycopods, in which the sporogones bear but one kind of 

 spore, producing monoecious prothalli only ; and the 

 Heterospores (Selaginella: and Isoetes), in which the 

 sporogone bears both microspores and macrospores. 



In the microspores of heterosporous Lycopods a single 

 cellule represents the male thallus, and appears a useless 

 appendicle to the antherozoid-producing cellules. The 

 macrospore germinates into two cellular masses,. corre- 

 sponding to the female thallus, which, although never 



