256 THE PHENOMENON OF 



cording to Nageli's* observations (on the formation of 

 the pollen of Lilium), the tetrahedral arrangement of the 

 four special mother-cells occurs also when they are 

 formed in the second generation, when the two primary 

 special mother-cells first produced are divided, not in the 

 same, but in a different direction, producing a decussating 

 arrangement of the two pairs.t On the other hand, it 

 follows from Hofmeister's description of the formation of 

 the pollen in Pinus,\ that four special mother-cells lying 

 in one plane may be produced by simultaneous formation. 

 But what seems to us of still more importance here, is 

 the observation, that not merely the various modes of 

 arrangement, but even the cases, which we should suppose 

 very essentially different in their mode of formation, of 

 primary and secondary, direct and indirect formation of 

 special mother-cells, are not exclusive of each other in 

 their natural occurrence, since they sometimes occur in 

 substitutive alternation in one and the same plant. 

 Nageli has named Altliaa rosea as a plant in which the 

 special mother-cells are formed in one generation (four 

 at a time), and also sometimes in two successive gene- 

 shape, with a longitudinal keel,) e. g., in Polypodium tulgare, Ceterach 

 qfficinarum, Asplenium Ruta muraria, Aspldium Filix feir.ina, &c. ; the latter 

 (producing the form of a triangular pyramid rounded at the base,) in Pteris 

 longifolia, serrulata, Allosonu crispvs, Notochleena marantte, Osmunda regalis, 

 &c. (H. von Mohl,' Verm. Schrift.,' 69, 70.) In the Mosses and Hepatic* 

 the tetrahedral arrangement seems to be predominant, h'kewise in the Lyco- 

 podiacese and Rhizocarpeae, in the latter of which this is indicated by three 

 short lines united in the form of a tripod, both on the small and imperfectly 

 developed large spores, (Mettenius, 'Beitrage zur Kenntniss der Rhizo- 

 carpeen,' t. i, of Salvinia.) In the Florides there occurs a third mode of 

 arrangement, in addition to the quadratic and tetrahedral, namely, the linear, 

 (Kiitz., 'Phyc. gen.,' p. 100,) "spermatidia quadrijuga." See, for instance, 

 t. 60, iv, Hypnophycus musciformis, and t. 62, iii, Calliblepharis ciliata. 



* Nagefi, ' Entwick. des Pollens,' p. 18, t. ii, f. 19, 20, 21. 



t The shape of the four secondary special mother-cells, (and consequently 

 of the pollen-grains or spores,) is in this case, however, different from that 

 in the tetrahedral arrangement through direct formation ; in the former case 

 the four portions, (as in the quadratic arrangement,) have the form of quarters 

 of a sphere, in the latter the form of a three-sided pyramid with a rounded 

 base. If the two modes of arrangement require to be distinguished, the 

 former may be called the decussate, the latter the pyramidal or tetrahedral 

 in a restricted sense. (See also Nageli, 'Neueren Akensysteme,' p. 190.) 



\ 'Bot. Zeitnng.,' 1848, 671. 



