116 



KNOWLEDGE 



[May 2, 1898. 



There would seem to have been three alternatives open 

 to the water plants which invaded the land. In the firstplace 

 they might perhaps accommodate themselves to living under 

 terrestrial conditions without undergoing much structural 

 change. So long as moisture is abundant enough at times, 

 a plant can, as it were, learn to do without it at others. 

 It can Hve through times of dryness, and complete its 

 life history when proper conditions are prevalent. Thus 

 the simpler forms of the green and the blue-green algie can 

 be found in salt water, in fresh water, and on damp earth, 

 trees, or brick walls ; and even such highly developed forms 

 as Chrodleptis and Pln/copcltis are distinctively land plants, 

 though retaining all their algal characters. One is liable 

 to forget that a very thin layer of moisture is suificient in 

 proportion to the actual size of these living cells : a con- 

 sideration of much importance in connection with the 

 reproduction of terrestrial cryptogams. 



Other algal forms seem to have accommodated them- 

 selves to earthly life, even under conditions of extreme 

 dryness, by entering into a mutual contract with certain 

 fungi, and establishing the type of plants we know as 

 lichens. This story is a biological romance in itself, but 

 for the present outside our line of study. 



The third alternative was the gradual change of the 

 structure of the plant, so as to render it more capable of 

 existence and propagation imder altered circumstances. 



In this process of accommodation of the plant to new 

 conditions it became advantageous to develop parts which 

 could penetrate the soil and absorb moisture and partial 

 nutriment from it, as well as to raise some parts above the 

 original level, to be sure of their reaching the sunlight. 

 Thus in time resulted the well-defined distinction of root, 

 stem, and leaf, with which we are naturally familiar. 



The simplest of terrestrial plants— using the term for those 

 distinctly modified for living on ground, as distinct fi:om 

 algffi — are the IJepaticce or Livernorts. They can be found 

 everywhere, on damp banks or rocks or tree-trunks, and 

 they present a beauty and variety of structure that makes 

 them a fascinatmg study to the microscopist. 



There are the wide-spreading lobes of the flat thallus of 

 Marchantia and its kin, to be found on moist banks every- 

 where ; and in mossy places in the springtime the slender 

 leafy branches of the ■hmi/ernuinnias grow and fruit with a 

 fresh luxuriance which renders them, in the opinion of 

 many, more attractive than their drier and hardier cousins, 

 the mosses. It is unfortunate that names of such Teutonic 

 clumsiness as Liverwort and Jungermannia should be 

 applied to so graceful a group ; but the necessities of 

 scientific nomenclature oblige us to retain them. 



So far as the vegetative part of the plant is concerned, 

 the different genera do to some extent bridge over the 

 space between the mosses above and the algse below. 

 Though in Marchantia the thallus has a structure so 

 specialized as to show at once how widely it is separated 

 from a simple seaweed, there are other forms, like Paccia 

 and Anthoceros, in which the thin, green, plate-like thallus 

 seems but little different from Coleochate. 



Yet all the forms included in the Hepatic a differ from 

 lower plants and agree with all higher crptyogams in the 

 fact that the most essential part of the reproductive system 

 is an " archegonium." So constant is this organ that the 

 liverworts, mosses, ferns, and fern allies are now often 

 grouped together under the name "Arclt^goniatce." 



An archegonium is a flask-shaped structure containing 

 the oijsphere. Its lower part is an oval chamber in which 

 the egg-cell lies, and is prolonged upward into a neck 

 composed usually of four columns of cells round a central 

 axis. As the egg-ceU becomes mature the cells of the 

 central axis degenerate, so that a canal is formed full of 



a mucilaginous material, which projects also at the top of 

 the neck. 



The antheridia are little round or oval bodies occurring 

 at the bases of the leaves of the same or a separate shoot. 

 Their interior cells divide up into numerous minute 

 antherozoids (or xpennatozoids) which are capable of swim- 

 ming by means of a pair of delicate ciha. They are, of 

 course, so minute that even a raindrop is amply sufficient 

 to enable them to reach the archegonia. When this 

 occurs some of them enter the mucilaginous cap at the 

 end of the neck, and, making their way down the canal, 

 effect the fertilization of the oosphere. 



Hepatics, then, differ from even the highest of thallo- 

 phytes in the fact that the egg-cell is, even before fertiliza- 

 tion, contained in a special structure intended not only for 

 protection, but also modified to assist in the process of 

 fertilization. 



It might be expected that this advance in the preliminary 

 stages would be followed by a corresponding elaboration of 

 the fruit structure, but such is not the case. In aquatic 

 types, such as Coleochate, the resting stage seems to be 

 a necessity to the oospore in view of the future free- 

 swimming life of the unprotected carpospores. 



In a land plant, on the other hand, the swimming powers 

 of the carpospores would be of little use in comparison 

 with the far wider distribution attainable by the help of the 

 wind. If the spores developed protective coats of their 

 own they would be capable of considerable resistance to 

 adverse conditions, and this appears correlated with a 

 decreasing necessity for a resting stage of the oospore. 



Accordingly, what takes place after fertilization is an 

 almost immediate enlargement of the fertilized egg-ceU, 

 and its commencing internal division into spores. At the 

 same time the cells below it grow down into close con- 

 nection with the tissues of the parent plant, forming a 

 structure known as the foot. So that the spore-bearing 

 generation or Sporophyte adopts the method of remaining 

 in connection with its parent plant till it is ready to liberate 

 its spores ; instead of, as in the algse, preparing itself to 

 keep alive for a time after the parent plant has died down. 

 In this way it differentiates itself into a foot which remains 

 in connection with the parent tissues, a seta or stalk which 

 grows upward, and a globular sporangittm or capsule carried 

 at its apex. The whole is thus not a fruit, but almost a 

 second or alternative plant dependent on the first. It has 

 no roots, leaves, or green colour (chlvrophi/ll), and therefore 

 cannot exist as an mdependent plant, but is nevertheless 

 on the road to become one.* 



The further stages in the life history differ, of course, 

 in different genera, but the main features are remarkably 

 uniform throughout the group. 



The neck of the archegonium withers after fertilization 

 of the egg-ceU. Its wall enlarges for a time with the 

 growth of the oospore, but finally spUts at the top, leaving 

 a sheath, termed the laginula, round the base of the up- 

 growing sporophyte. 



The tissues of the sporangium itself become differentiated 

 into an outer two- layered wall and an inner cell-mass or 

 archesporium . It is in the further development of the latter 

 that we meet with the most striking difference from the 

 corresponding organ in algie, and the most evident modifica- 

 tion for terrestrial conditions. The cells divide up into a 

 large number of smaller ones, and while some of these 

 develop into spores, the remainder change into long sterile 

 threads termed elaters. The latter are long filaments 



* In one very interesting group of the Jungermanniacfcc, the 

 archegonia are carried in a sac-like structure hanging down from 

 the stem, and in some cases this may bury itself in the soil and even 

 attempt to root itself. 



