SPECIAL MORPHOLOGY: SEXUAL PLANTS. 201 



One peculiarity I often met with in the subterraneous stems. The 

 somewhat elongated cells which bound the air-passages, at a late stage 

 begin to develop cells in their interior. These penetrate particular 

 places in the wall of the mother cell, and project as utricles into the air- 

 cavities, then expand into a perfectly globular form, become cut off by 

 constriction, and thus fill the air-space up a second time with lax globular 

 cellular tissue. I cannot yet decide whether this is a diseased or a 

 regular structure. 



B. SEXUAL PLANTS (PL gamica). 



113. The sexual plants are at once characterised, as a large 

 and connected division of the vegetable kingdom, by the peculiar 

 manner in which a new individual is formed, and by the double 

 and essentially distinct organs which are required for this purpose. 

 Firstly, they develope four cells, clothed by a peculiar membrane, 

 within a mother-cell (sporangium of the Agamce), which becomes 

 absorbed subsequently, so that the former, when perfectly mature, 

 lie free in a little sac composed of cells (sporocarp of the Agamce). 

 This sac is called the anther (anther a\ the spores are called pollen 

 or pollen-granules (granula pollinis), and the special membrane by 

 which they are clothed is the external pollen-membrane. Secondly, 

 they produce a cellular body free in any case at the apex, of oval 

 or elongated form, in which one cell becomes so much enlarged 

 that it causes the absorption of a part of the surrounding sub- 

 stance, and thus a considerable cavity is produced in the body. 

 This body is called the seed-bud (or ovule) (gemmula) ; the great 

 cell is the embryo-sac (sacculus embryoniferus}. The sac contains 

 cytoblasterna, from which (excepting in the Rhizocarpeci) new cells 

 are formed, gradually filling the embryo-sac, until, as sometimes 

 happens, the growing embryo again displaces them. The develop- 

 ment of the new plant proceeds by the expansion of the cells of 

 the pollen-granules into a tube, which under favourable circum- 

 stances penetrates to the embryo-sac. The other end of the pollen- 

 tube dies away while the extremity in the sac developes new cells, 

 which become arranged into the form of the rudimentary plant, 

 the embryo. 



I have here only drawn attention to those points admitted with re- 

 spect to the Phanerogamic plants by all the best observers of recent 

 times. (With regard to the Rhizocarpece, see the special explanations.) 

 I have here displayed the essential facts, namely, the analogy of the 

 course of formation and the nature of the pollen with those of the 

 spores of the Agamce, and the wholly*sinrilar conversion of the pollen 

 into a tube, of which one end (though it must be freely admitted that 

 we do not know precisely how) forms new cells, which gradually arrange 

 themselves and form the new plant, while the other end dies away ; com- 

 paring this with the germination of Mosses, Ferns, &c. : and, while I 

 have distinctly marked this comparison, it will at once have become evi- 

 dent what new phenomenon is met with in the sexual plants, namely, 



