ACROGENS. 5 1 



Class II. ACROGENS. 



PsEUDOcoTYLEDONE^. Agavdh, Aph. 72. (1821K— Heteroxemea, Fries, Spst. Orb. Veg. 1. 30. (1825). 

 AcROBRYA, Mohl. in Mart. PI. Crypt, p. bQ; Endlich. Gen. p. 42. (in Prt7-<.)— Acrogen^, 

 Ad. Brongn. Enum.p. xiii. (1843). 



With this class a great advance In structure is aecomplislied. The sim- 

 plicity which is so remarkahle in Thallogens is exchanged for a complicated 

 apparatus of many kinds. All the species have stomates or hreathing-pores 

 on their surface : in the great majority there is a distinct stem and leaves, 

 the latter of which are always arranged with perfect symmetry ; and in 

 those species which approach Thallogens, (as the Crystalworts, which stand 

 close upon Lichens) the thallus has all the texture of leaves, although a 

 separate stem is refused to them. There is, however, no trace of flowers, 

 properly so called ; and yet in the involucre of many Liverworts, and in 

 the spore-cases of Mosses, an arrangement of leaves occurs, which appears 

 to he the forerunner of the flowers of more perfect plants. Sexes, how- 

 ever, are wholly missing ; that is to say, nothing can he found which 

 resemhles the anthers and pistil of flowering plants, except in some vague 

 external circumstance : there is no evidence to show that anj^ one order of 

 Acrogens possesses organs which require to be fertilised the one by the 

 other in order to eftect the generation of seeds. Hence those reproductive 

 bodies of Acrogens which are analogous to seeds are called spores. Mr. 

 Griffith takes, however, a very different view of this question, and assigns 

 true sexes to Acrogens. 



He thinks it probable that we have at least three modifications of the 

 phenomenon of fecundation "among the higher acotyledonous plants. In 

 one the male influence is applied to the apex of a pistillum, in the second 

 to a nucleus without the intervention of a pistillary apparatus. In the 

 third the male influence is exerted on a frond itself, and is followed by the 

 development of the young capsule from a point in the substance of the frond 

 corresponding to and sometimes distant from the place to which the male 

 influence has been applied. This is founded on observations made on 

 Anthoceros in 1836, from which it would appear that the place of exsertion 

 of the future capsules is pointed out by a slight protuberance, over the apex 

 of which a flake of matter like the so called male matter of Musci and 

 Salvinia is spread, sending down to some distance within the frond a 

 tube-like process, which causes the dislocation of the ceUs of the tissue with 

 which it comes into contact. The future capsule is stated in his notes not 

 to be appreciably pre-existent, and its situation is only pointed out by a 

 bulbiform condensation of the tissue of the frond. The young capsule 

 during its development ascends along the same line, and pushes before it a 

 corresponding cylindrical body of the tissue of the frond, the calyptra of 

 authors." But, it seems to me, that this very complexity of action is more 

 like variations in self-propagation, than phenomena of fecundation, which, 

 among the plants in which that action certainly takes place, is subject to 

 no such modifications. 



A large number of Acrogens have no true spiral vessels, which are con- 

 fined to" the more highly developed forms, such as Ferns, Clubmosses, and 

 Horsetails ; but there is a very general tendency to the production of spiral 



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