624 PHYSIOLOGY. 



order, in different species of the same genus, e. g. Viola, and 

 even in the same species, e. g. Fuchsia, Primula, Mimulus. The 

 most frequent cases of agreement in allied plants occur when 

 they possess compouml pollen-grains (fig. 602, b, d), consisting 

 of a number of pollen-cells permanently coherent together. 

 The most striking cases of this are those offered so abundantly 

 in Orchidaceae and Asclepiadaceae as to form valuable systematic 

 characters in these Orders. These pollen-masses or pollinia con- 

 sist either of the entire mass of pollen of an anther-cell, or of a 

 half, quarter, eighth, or smaller fraction, so numerous in some 

 genera as to appear like granules merely coarser than ordinary 

 pulverulent pollen. 



Formation of Pollen. The existence of pollen-masses and compound 

 grains is readily accounted for by the history of the development of 

 pollen, which agrees in the main points with that of the spores of Mosses 

 &c. (p. 429). The parenchyma in the central region of each lobe of a young 

 anther presents two perpendicular rows of cells, one corresponding to each 

 of the four primary loculi, different in character from those which 

 are to form the walls. The cells in these series multiply by division to a 

 considerable extent ; and ultimately each forms a free cell from its whole 

 contents the parent cell of the pollen. These are set free by solution of 

 the walls of the parenchymatous framework in which they have been de- 

 veloped, and they then lie as loose cells in the loculi or chambers of the 

 anther thus formed. Each parent cell divides into four chambers ; and 

 each of these chambers (special parent cells) produces a pollen-cell, in the 

 case of simple pollen-grains set free by the solution of the special parent 

 cell. In quaternary pollen (fig. 602, d) the membranes of the special 

 parent cells are not dissolved, and thus the pollen-cells are held together 

 in fours ; and the more complex conditions arise from the membranes of 

 the parent cells of anterior stages persisting sufficiently to hold their pro- 

 geny together. The mode of formation of the pollen in the special parent 

 cells is by some attributed to cell-division ; but the more generally adopted 

 view is that it is formed bv free-cell formation. The nucleus of the 

 parent cell divides into two ; "between these two a quantity of granules of 

 protoplasm are aggregated together in a direction across the parent cell : 

 these granules are suddenly seen to be divided by a line, the first indica- 

 tion of the cell-wall between the two cells so produced ; these two again 

 subdivide ; and thus four pollen-cells are ultimately found in one parent 

 cell. The pollen-masses of the Asclepiadacese, and perhaps of some 

 Orchidacea3, result from a different process : in these the outer layers of 

 the primary parent cells do not develop cells in their interior, but become 

 conjoined into a cellular pellicle forming a sac or purse enclosing all the 

 poll en-grains formed within. 



The pollen-cells acquire their cuticular coat after they have become 

 free ; but part of the material of this structure appears to be derived from 

 the dissolved membranes of the parent cells. 



Zostera presents a remarkable exception to the usual character of pol- 

 len-grains, the cells here having the form of short cylindrical filaments 

 with but one coat, i. e. without a cuticular layer. In these a rotation 



