192 THIRD GROUP. 



at a very early stage in the development of the sporangium. The mature sporangia 

 are roundish capsules of very simple structure and small size. A sporangium when 

 half developed consists of three parts : i. an inner [sporogenous) mass of tissue which 

 afterwards becomes the mother-cells of the spores {sporocytes) ; 2. one or more layers 

 of tabular cells, the tapetal cells or tapetum, which form an investment round the 

 sporogenous cells ; 3. the ivall of the sporangium formed of one or more cell-layers. 



From what has now been said it is plain that the sporangia of the Vascular Cryp- 

 togams are physiologically but not morphologically equivalent to the sporogonium of 

 the Mosses ; the latter represents by itself the whole sporophyte of the moss-plant, 

 while the sporangium of the Vascular Cryptogams is a relatively small outgrowth 

 usually of a leaf of the sporophyte which consists of stem, leaf and root. 



The way in which the spores are formed in the mother-cells agrees with the 

 corresponding process in the Muscineae ; the mother-cells separate from the previously 

 connected tissue and divide into four spores, a bipartition of the nucleus preceding the 

 division into four in each cell. The distinction of macrospores and microspores in 

 the Salviniaceae, Marsiliaceae, Selaginelleae, and Isoeteae does not make its appear- 

 ance till after the division of the mother-cell into four parts, the parent-cells of both 

 kinds of spores being exactly alike up to that time. 



It was Hofmeister who in the year 1851 first showed that the sporogonium of 

 the mosses, the moss-fruit as it is usually called, is by its position in the alter- 

 nation of generations the equivalent of the entire plant with leaves and roots, which 

 bears the spores in the Vascular Cryptogams ; he it was also who showed how the 

 Selaginelleae and Isoeteae are connected with the Coniferae, and these two discoveries 

 have proved to be among the most important and the most fruitful in results that 

 have ever been made in the domain of botanical morphology and classification. 



Other enquirers, who will be noticed hereafter, have added so much by their 

 many and exhaustive researches to our knowledge of the Vascular Cryptogams that 

 they are at the present time one of the most thoroughly explored groups in the 

 vegetable kingdom. These investigations into the development of the sexual organs, 

 of the embryo, of the sporangia, of the spores and the germination of the spores have 

 brought to light so great a community of characters, the result of community of 

 descent, that it is now possible to submit the whole to comparative treatment. But 

 this presupposes a more general knowledge of these plants, and we will therefore 

 give an account of the facts bearing on their relation to other groups when describing 

 the several families \ 



CLASSIFICATIOlSr OP THE VASCULAR CRYPTOGAMS. The connec- 

 tion between the different sections of the Vascular Cryptogams still requires elucidation 

 on many points of detail. The division of them into Isosporous, or those with only 

 one kind of spore, and Heterosporous, or those with macrospores and microspores, 

 has proved to be artificial, since isosporous and heterosporous forms are found within 

 the same cycle of alliance ; in the Ferns, for example, the Polypodiaceae and others are 

 isosporous, the Salviniaceae heterosporous, and in the same way the isosporous Lyco- 

 podiaceae are nearly related to the heterosporous Selaginelleae. The Ferns in the 

 narrower sense, that is, excluding the Marattiaceae and Ophioglosseae, but including 



Vergleichende Untersuchungen, p. 139. 



