.8^. A NEW GROUP OF FLOWERING PLANTS. 135 



walls ; single longitudinal rows grow less rapidly and form sterile 

 trabeculae separating the rows of fertile cells. The latter give rise to 

 a few tapetal cells by transverse division towards the outer wall and 

 one sporogenous cell, the mother-cell of the macrospores, which 

 becomes rounded off and grows considerably, destroying and using 

 up the tapetal and neighbouring cells, till it thus comes to lie in a 

 cavity of the sporangium, where it divides into four daughter-cells, the 

 macrospores. 



The germination of the male or microspore somewhat resembles 

 that of the Rhizocarps. 



The macrospore in Isoetes a few weeks after being set free from 

 the decaying macrosporangium, becomes filled with cells, which are 

 at first naked, but when the spore is quite filled become invested 

 with a cell-wall. The prothallium then occupies the whole interior 

 of the spore, which now bursts at the apex, and one or, if necessary, 

 more archegonia are developed on the exposed portion of the 

 prothallium. 



The alternation of sexual and asexual generations is evident in all 

 these families classed together in the great group of Vascular Crypto- 

 gams. In every case the sexual generation, though it may not escape 

 from the spore, and sometimes not even get quite free from the sporan- 

 gium, as in Salvinia, is yet distinct from the always conspicuous 

 asexual plant, with its evident differentiation into root, stem, and 

 leaf. It was not until Hofmeister published his "Comparative 

 Researches," in 1851, that the relation became clear between the 

 higher plants, the Phanerogams, or Seed-plants, and the Vascular 

 Cryptogams. We now understand that the same principle runs 

 through the life history of all, and the accurate correlation of the 

 organs in the two great classes has thrown a much clearer light on 

 the principle of classification. Phanerogams are characterised by 

 producing a seed, and it is by the proper comprehension of the 

 development of the seed that we are able to understand these 

 relations- 

 Phanerogams are heterosporous, and the indication of sex has 

 penetrated far into the asexual generation. There are not only two 

 sets of sporangia, but two sets of modified leaves to bear them. 

 Those which bear the microspores are the stamens, while the carpels 

 have the same relation to the macrospores. Stamens and carpels 

 are commonly associated in the flower, a stru^cture unknown in the 

 Cryptogams, but developed in the Phanerogams to ensure the 

 protection of the " essential " sexual parts, and the proper per- 

 formance of their functions. The method of production of the 

 microspores or pollen-grains follows a general plan similar to what 

 obtains in the microsporangium of Vascular Cryptogams like Isoetes. 

 The pollen-sac, or sporangium, arises from a group of cells sometimes 

 on the underside of a leaf-like stamen, as in Conifers, sometimes, as in 

 Angiosperms, several together, forming the anther at the apex of the 



