XV 



THE DEVELOPMENT OF THE FLOWER 



199 



are formed. The principal difference, then, is the division by a 

 wall of the cell into two after the division of the nucleus. (Fig. 

 207). The pollen grains of dicotyledonous plants are formed 

 by free cell formation, but in monocotyledonous plants by a 

 method which comes between this and vegetative division, 

 (p. 89). 



The Structure of a Pollen Grain. The pollen grain is 

 at first surrounded by a very thin cell-wall which with age in- 

 creases in thickness. The outer layer becomes cuticularised 

 and forms the extine', the inner layer consists of cellulose and 



FIG. 207. Diagram illustrating the deve- FIG. 208. Pollen grain. E, extine ; 

 lopment of pollen in a monocotyle- I, inline ; N, vegetable nucleus ; 



donous anther. G, generative nucleus. (Dia- 



grammatic.) 



forms the intine. The shape of the grain varies in different 

 plants. The extine, too, may be raised into knobs, spines, and 

 ridges or be perfectly smooth. Most pollen grains have thin 

 places in their extine and out of one of these places the intine 

 grows to form the polle?i tube. 



The interior of the grain is filled with granular protoplasm, in 

 which two nuclei may with great difficulty be made out. The 

 smaller nucleus is the generative one, and the larger one the 

 vegetative nucleus. When the pollen grain is placed under suit- 

 able conditions (p. 152) germination takes place, and the intine 

 breaks through the extine to form a long tube called the pollen 

 tube, which is of service in carrying the generative nucleus to the 

 ovule. 



EXPT. 179. Cut a transverse section from the filament of any well- 

 developed stamen. Mount in water and examine, first with a low, then 

 with a high power. Note 



(i) The outside epidermal layer. 



(ii) A few rows of parenchyma within the epidermis which represent 

 the cortex of the petiole. 



(iii) One or more vascular bundles in the centre of the section ; these 

 are continuous with the bundles of the stem, and bring nutritive 

 materials to the anther and pollen grains. 



