364 FOURTH GROUP. SEED-PLANTS. 



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

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

 microspore. In most Dicotyledons the process is somewhat different ; the division of 



the pollen-mother-cell is not fol- 



A 



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 time 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. Althaea, rosea. A division of the mother-cells of the 

 pollen into four. F and G a tetrad, in which the walls of the special 

 mother-cells are bursting under the influence of water and allowing the 

 protoplasm of the young pollen-grains to escape. Ha. mature pollen-grain 

 seen from without and magnified the same number of times as the other 

 figures. 



sK v 



I. JL. 



FIG. 292. Leucojum aestivtim. Pollen-grains (microspores) with vegetative cells. / shows the pollen-grain after 

 division into the vegetative cell v and the larger cell with the nucleus si. 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 contents not 

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



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

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

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



