364 



FO UR TH GR UP. — SEED-PLANTS. 



lying beside one another (Fig. 289), and as each cell divides again, the four ' special 

 are formed, and the contents of each of these becomes a pollen-grain or 

 In most Dicotyledons the process is somewhat diiferent ; the division of 



the pollen-mother-cell is not fol- 

 ■^ lowed by the formation of a firm 

 wall of cellulose, but the daughter- 

 nuclei divide again at once in 

 intersecting planes. The four 

 nuclei then take up a position to 

 one another answering to the four 

 angles of a tetrahedron, and it is 

 only then that firm walls are 

 formed, which divide the mother' 

 cell into four daughter-cells dis- 

 posed tetrahedrally. Ridge-like 

 projections appear first on the in- 

 ner wall of the pollen-mother-cell 

 (Fig. 291 A, D) corresponding in 

 position to the cell-plates between 

 the four nuclei. Then partition- 

 walls form very rapidly between 

 the separate cells and attach 

 themselves to these projections. 

 The pollen-mother-cell thus di- 

 vided into four is called a tetrad. 



The mass of cellulose round 

 each separate cell of the tetrad is 

 differentiated into concentric sys- 

 tems of layers (the ' special mother- 

 cells'), and these are surrounded by 

 the common layers which encompass the whole tetrad (Figs. 291 E, 294) ; if the tetrads 

 lie some lime in water, the layers often burst, and the protoplasmic bodies of the 

 young pollen-grains escape through the rent and become rounded off into a spherical 



Fig. 291 Altl lea 10 n A-F d s n f 

 pollen into four F and G a tetrad m wh h tl ( 

 mother-cells are burst ng under tl e influence of 

 protoplasm of tl e y un? pollen gra ns to escape h 

 seen from without and magnified the same number 



illow ng the 

 ! pol en griin 

 as the other 



I-IG. 1-92. Leiicojwn aestivum. Pollen-grains (microspores) with vegetative cells, /shows the poilen-grain after 

 division into the vegetative cell v and the larger cell with the nucleus sk. In // the vegetative cell has become detached ; 

 o is the vegetative cell after treatment with osmic acid. /// a pollen-grain which has put out a tube (the confnts not 

 shown) with two similar nuclei After Elfving. Magn. 400 times. 



form (Fig. 289 F//and Fig. 291 F, G). Soon after the conversion of the pollen- 

 mother-cell into a tetrad, each of the daughter-masses of prolophsm invests itself 

 with a new and at first verv delicate cell-wall, which is not connected with the inner- 



