EEJtIVENESCENCE IN NATURE. 255 



pollen-grains of the Phanerogamia and the spores of the 

 Cryptogamia, the formation of four daughter-cells in each 

 mother-cell appeared to be a case of very frequent occur- 

 rence, widely diffused in the condition of transition to 

 fructification. From more recent researches, however, on 

 the processes which precede the formation of both the 

 pollen-grains and the groups of four spores,* in the mother-" 

 cell, the nature of the formation is not so simple as it at 

 first appeared. The pollen-grains and spores are not 

 formed immediately in fours in the mother-cell, but singly 

 in special mother-cells, so that the origin of the number 

 four is to be examined in the latter. According to 

 Nageli's researches, this takes place in two ways.f The 

 mother-cell either divides at first into two daughter-cells 

 (primary special mother-cells), which then each divide again 

 into two (secondary special mother-cells) ; or the mother- 

 cell divides immediately into four daughter-cells, in which 

 case, consequently, only one generation of special mother- 

 cells (merely primary) is formed. Only the second case 

 belongs to the present section of our subject. From 

 researches hitherto made, it seems to follow that in the 

 first mode of origin the group of four (secondary) special 

 mother-cells are usually placed in one plane ; in the latter 

 the four (primary) special mother-cells in two planes, 

 or in the order of the angles of a tetrahedron. If this 

 relation of the arrangement to the mode of origin were 

 constant, it would be an easy means of ascertaining the 

 direct or indirect formation of the group of four special 

 mother-cells, at a later epoch, from the arrangement, nay 

 even in many cases from the form (in a great measure 

 dependant on the original position) of the spores or 

 pollen-grains. I But this relation is not constant. Ac- 



* Here belong; the formation of the species of the Ferns, Mosses, and 

 Hepaticffi ; both kinds of spore-formation in the Lycopodiaoese, Isoetacese, 

 and Rhizocarpese ; and one kind of spore-formation in the Moridese. 



f 'Zur Entwick. des Pollens der Phanerog.,' 1842. 



% Quadratic (situated in one plane) position and elongated form of the 

 pollen-grains occurs principally in the Monocotyledons ; tetrahedral position 

 and roundish form almost universally in the Dicotyledons, (Mohl, 1. c.,p. 34.) 

 Both cases are likewise repeated in the spores of Perns, the former (longish 



