HENDERSON'S HANDBOOK OF PLANTS. 



ACR 



terminates the branches. Unfortunate- 

 ly, even in the same genus, as Fissidens, 

 species with lateral and terminal fruit 

 occur, so that the distinction is not with- 

 out grave exceptions. 



Acrogens. A large and most important 

 division of Cryptogamia, distinguished for 

 the most part from Thallogens, as Fun- 

 guses, Sea-weeds, and Lichens, by their 

 herbaceous growth, the presence of leafy 

 appendages, which are frequently fur- 

 nished with stomates, the different mode 

 of impregnation, and the presence of vas- 

 cular tissue. A few acrogenous Liver- 

 worts have the habits of Lichens, but 

 differ totally in structure. The most im- 

 portant distinction, however, undoubtedly 

 is, that the impregnation takes place 

 somewhat after the manner of Phseno- 

 gamia, by an impression made upon the 

 contents of the embryonic sac, and not 

 upon the spore itself, as is decidedly the 

 case among Thallogens where the mode 

 of impregnation has been ascertained, as 

 in Algce. In Characece alone the spore 

 seems to be immediately impregnated, 

 though even in this case it is uncertain 

 whether impregnation does not take 

 place before the spore is perfected. In 

 Mosses, Liverworts, and Ferns, the spore 

 after germination produces at first either 

 a web of threads, a solid mass, or a mem- 

 branous expansion, (prothallium.} In 

 the two former a distinct plant arises 

 from the threads, with frequently sym- 

 metrical leaves, and on these plants urn- 

 shaped organs are produced (called 

 archegones) analogous to pistils, which 

 contain at their base a cell which, after 

 impregnation, produces the proper fruit. 

 In perennial species a fresh crop of ar- 

 chegones may be produced in two or three 

 successive years, which require a distinct 



ACR 



act of impregnation for the development 

 of the capsules. In Ferns and their 

 allies, on the contrary, the archegones 

 give rise to a new plant, which for one 

 or for many successive years produces a 

 fresh crop of fruit without further im- 

 pregnation. The result of impregnation 

 in the two cases, then, is quite different. 

 In Mosses the whole plant is, as to func- 

 tions, a prothallium; in Ferns, merely 

 the membranous expansion immediately 

 produced on the germination of the 

 spores. Further details may be reserved 

 for each successive group. In those spe- 

 cies of Fungi, as Puccinia, Podisoma, etc., 

 where a prothallium is produced, it has 

 the nature of a spore, and germinates in 

 the same manner. 



As regards the tissues, it may be ob- 

 served that the stem of many Acrogens 

 contains distinct vascular tissue. In Jun- 

 gennannice, where such tissue is rare else- 

 where, it almost universally accompanies 

 the spores. In Mosses, as in fiphagnum, 

 there are sometimes distinct spirals in 

 the cells of the leaves. The vascular tis- 

 sue in most of the higher Cryptogamia is 

 scalariform; but in Iso'etes and Equisetum 

 it is annular, with transitions to short 

 spirals ; while in Selaginella and Lycopo- 

 dium there is a transition from short 

 spiral and reticulated cells to elongated 

 cells, which may be called spiral vessels. 

 In the stem of Sphagnum there is tissue 

 closely resembling the glandular tissue 

 of Conifers. The spiral coats of the spores 

 in Equisetum will be noticed hereafter. 

 The impregnating bodies or spermato- 

 zoids have always nagelliform append- 

 ages, sometimes nuich more highly devel- 

 oped than in the spermatozoa of animals. 

 The principal divisions of Acrogens are 

 the following: 



