1364 A TEXTBOOK OF THEORETICAL BOTANY 



shall describe shortly, and the young grains are left suspended in the result- 

 ing colloidal matrix, from which they absorb nutritive substances. 



The pollen grains of Carex and some allied genera are singular in that 

 they consist of the mother cell itself. The nucleus undergoes the ordinary 

 meiotic divisions and then three of the four nuclei thus formed pass to the 

 apex of the bluntly conical grain and there abort, disappearing into the 

 intine. The wall of the pollen grain which develops is, therefore, the wall 

 of the pollen mother cell. 



Chemical analysis of mature pollen shows the following range of com- 

 position: 



Protein from 



Fats from 



Carbohydrates from 



Ash from 



Water from 



The protoplast of the grain in its earlier stages is only a hollow shell, 

 with one large central vacuole. The nucleus increases in size and density 

 and the cytoplasm gradually increases in bulk and finally occludes the vacu- 

 ole, an event which marks the maturity of the grain. Mature grains contain 

 large amounts of starch or in some species fatty material, probably absorbed 

 from the tapetum. 



Some stress has been laid on the diflference between starchy and fatty 

 pollen and it has been claimed that starchy pollen is commoner in cold 

 latitudes. Systematic comparisons between plants in Lapland and in 

 Germany do not seem to support this idea. In the worst seasons in Germany 

 the number of species producing fatty pollen increased, though most starchy 

 pollen species remain unchanged whatever the season. Starchy pollen 

 species belong to several distinct ecological types and there is no apparent 

 ecological relationship between them. 



Many pollens lose their starch grains at anthesis. In others the grain is 

 still starchy when shed and the starch is solvated in the pollen tubes, but^at 

 very unequal rates. There is usually a higher osmotic potential in the pollen 

 tube than in the surrounding stylar cells and solvation of starch may promote 

 this. 



A curious variation is shown by Lythrum salicaria, where the green pollen 

 in the heterostylous flowers is starchy and the yellow pollen is fatty. The 

 former needs a higher sugar concentration for germination than the latter. 



Some heterantherous flowers produce both " fodder pollen " with a 

 high starch content, which is collected by bees, and fertilization pollen. 

 In most species the former needs a higher concentration of sugar than the 

 latter for germination, but in heterantherous species of Cassia the fodder 

 pollen also requires the addition of diastase, which appears to call out in- 

 ternal formation of diastase. Otherwise the starch in the grain cannot be 

 mobilized. 



There are two main types of behaviour of the tapetum. One is the forma- 



