626 



PHYSIOLOGY. 



which develops into the pol- 

 len-tube; i, the intine; q, 

 lowermost cell of the 

 male prothallus in contact 

 with the intine (according 

 to Strasburger, this is no 



of free-cell formation ; and they establish an homology with the mother cells 

 of the antherozoids of Cryptogams. The processes 

 are somewhat complex ; but if the lamination of 

 the cuticle and cell-wall be borne in mind (p. 520), 

 and the differentiation of the protoplasm into por- 

 tions of varying density (p. 495) and crystalloid 

 form be remembered, the stages of the i process 

 will be more readily understood by the student. 



Tchistiakoff's researches show that the pollen 

 of these plants is divisible into two groups, those 

 with and those without vesicular protrusions 

 or air-bladders. In both cases the mode of for- 

 mation of the extine is identical. In both cases 

 it consists of two layers j but where there are no 

 air-bladders the two layers of extine are formed 



simultaneously. In the opposite case they are Pollen of Bpruce Fir> after 

 formed in succession, there being at first between Schaeht : bl, the vesicular 

 the two an interspace filled with gelatinous fluid, 

 which absorbs much water by endosmosis. The 

 interspaces therefore become much distended, 

 and ultimately form the air-bladders attached to * 

 the grain, the fluid contents disappearing. The 



net-like markings on these bladders are due to cell but merely a fis8Ure) . 

 the remains of protoplasmic threads adherent 

 to the extine. As to the division of the pollen-cells and formation of a 

 male prothallus, M. Tchistiakoft* admits three types : 1, that of Ciipressus 

 and other genera ; 2, that of Larix &c. ; 3, that of Abies &c. In all cases 

 the starch in the cells becomes dissolved, the cell-wall (intine) swells up 

 and becomes more hygroscopic, so that by its distention it throws off the 

 cuticular extine, when the pollen-tubes begin to grow or germinate. 

 While these changes in the intine are taking place, the outermost por- 

 tion of the protoplasm becomes developed into a separate layer of globules 

 or crystalloid masses surrounding the whole or only a portion of the 

 plasma or cell-contents. The formation of the new cells may take place 

 simultaneously with, or after, the differentiation of this peripheral layer 

 of protoplasm, which, in the latter case, takes no part in the division. 



In the Oupressus or Thvja type, the pollen-cells either do not divide, or 

 each divides into two cells. The starch is dissolved, the nucleus divides 

 into two : of these, one becomes more homogeneous, denser, and more 

 transparent than the other, and resembles the homogeneous beak of the 

 zoospores of Algae. Both may be surrounded by the layer of protoplasmic 

 globules just described, or one only. 



In the Larix type, instead of two subdivisions of the plasma only, the 

 two first formed divide again. In the case of the last subdivision a por- 

 tion of the plasma becomes denser and more homogeneous, as in the 

 former case. In Pinus two or three subdivisions take place, the cells so 

 formed constituting a suspensor, the cell-division being preceded by 

 changes in the protoplasm, as in the case of Thvja. The third cell is 

 formed subsequently to the others, and is of a hemispherical form, sepa- 

 rated by a partition from the second cell of the suspensor, but having no 

 direct communication with the cell-w T all or intine ; nevertheless the mode 

 of formation of the partition is the same- That portion of the protoplasm 



