POLLEN-GRAINS OF GTMNOSPEEMIA. 627 



in contact with the partition separating it from the second cell of the sus- 

 pensor now secretes a cell-wall over the hemispherical portion of dense 

 homogeneous protoplasm found in that situation, and the new cell is 

 formed. 



In the Abies type the two first cells of the suspensor are formed as 

 above indicated ; but the third is formed by true free-cell formation, and 

 is quite detached from the suspensor. In the early stages it is like that of 

 Pinus ; but subsequently it may become completely isolated and divided 

 into secondary and tertiary subdivisions, often in a spiral direction. In 

 the germination of the pollen-tube, the intine of the larger of the two 

 cells into which the primitive one divides alone forms the tube. In Pinus 

 there may be more than one pollen-tube, in either case very large. In it 

 is sometimes formed, by free-cell formation around a nucleus, a large cell, 

 which becomes ultimately liberated by the absorption of the walls of the 

 pollen-tube. 



The new cells formed by free-cell formation and attached to the sus- 

 pensor, as well as those formed freely in the pollen-tube in the course of 

 its formation, are to be considered, according to this author, as the rudi- 

 ments of the mother cells of the antherozoid, the presence of which 

 Hofmeister had previously suspected. The cells of the suspensor cor- 

 respond precisely to the cells of the male prothallus of Isoetes. 



In the Phanerogams, when a pollen-grain falls upon a stigma in 

 its proper or " receptive " state (known by the presence of a sac- 

 charine secretion), the inner coat is protruded in the form of a 

 blind pouch (fig. 602, Ti) from one or more of the pores or slits 

 of the pollen-cell itself, and, nourished by the stigmatic secretions, 

 grows into a tube of great tenuity, which makes its way through 

 the loose stigmatic cells, and passes down the canal of the style 

 into the cavity of the ovary, there following the course of the pla- 

 centas when the ovules are numerous. 



In the Gymnosperms the pollen-grains fall at once upon the 

 ovules and pass into the micropyle, sending down their pollen- 

 tubes (here developed from one of the daughter cells, which pene- 

 trate through the proper coat of the pollen-cell) into the sub- 

 stance of the nucleus of the ovule, towards the deep-seated 

 embryo- sac. 



The formation of imperfect pollen-tubes may sometimes be caused by 

 placing pollen-grains in syruny fluids ; but when they are placed in dilute 

 sulphuric acid &c. the extrusion of the inner coat which results is mostly 

 a process of mechanical expansion, and the projecting pouches soon burst 

 and discharge the contents of the cell, owing to endosmotic action. 



Ovules of Phanerogamia. 



The ovules of Phanerogamia are all constructed according to 

 some modification of one general plan, which has been already 

 described (p. 137). In the succeeding paragraphs some further 

 details as to its history may be given. 



2s2 



