POLLEN: LTS DEVELOPMENT AND USE. 



339 



form four, each one surrounded by a cell-wall, but all still in- 

 closed by the wall of the mother-cell (Fig. 1). On further growth 

 the wall of the mother-cell is ruptured, and the daughter-cells 

 escape as free pol- 

 len grains. The wall 

 of the mother-cell 

 is then either ab- 

 sorbed, or remains 

 in the form of 

 threads between the 

 grains, or as viscid 

 matter on the out- 

 side of the grains. 



Thus pollen grains are daughter-cells, which have been devel- 

 oped from a primal mother-cell. Each grain is made up of three 

 parts. There is an outer wall (extine), an inner wall (intine), and 

 the fluid contents (fovilla). The extine is often marked with lines, 

 points, or grooves ; the intine is generally smooth and regular, and, 

 even when the extine is studded with points, the intine does not 



Fig. 1. a, Mother-cell, with two nuclei ; i, mother-cell divided 

 into two cells ; c, mother-cell divided into four cells. 



Showing marks on extine. With extine removed. 



Fig. 2. Hollyhock 



Fig. 3. Pollen of 

 CEnothera. 



line the inside of these points but extends over their bases (Fig. 2). 

 In cases where there are projections at different points, as in the 

 evening primrose {CEnothera, Fig. 3) and others, the intine be- 

 comes thickened, and the extine is very much thinner. In the 

 melon (Fig. 4), where the extine has pores pro- 

 vided with lids, the intine at these points is con- 

 siderably thickened, and in growth 

 pushes the cap off. When, again, 

 a pollen grain appears marked 

 with reticulations and spaces, as 

 in Pancratium (Fig. 5), these are 

 regarded as thinner places in the 

 extine rather than special mark- 



Fovilla. 



mgs. 



Fig. 4. Melon. 



Fig. 



It is a well-known fact that the 

 pollen of anemophilous or wind- 

 fertilized plants differs markedly 

 from that of entomophilous or in- 

 sect-fertilized plants. In the former case it is dry and powdery, 

 probably having this quality from the entire absorption of the 



5. Pancra- 

 tium. 



