CHAPTER XXV 



ANGIOSPERMAE : SEXUAL REPRODUCTION 



The subject-matter of this chapter will be the development and structure of 

 pollen grains and ovules; the gametophytic structures they produce; the 

 phenomena of fertilization and the post-fertilization changes which ensue, 

 including the growth of the embryo. The characters of fruits and seeds 

 as matured structures and the germination of seeds will be dealt with in the 

 following chapter. 



POLLEN 



The formation of the archesporium from the hypodermal layer of cells 

 in the voung anther has already been mentioned (p. 1 197). The number of 

 hypodermal cells which develops into archesporial cells is very variable. 

 Sometimes the whole of the hypoderm in each lobe develops, more generally 

 only a short, transverse row of cells is transformed into archesporium and in 

 Malvaceae and in many of the Compositae there is only one such cell. 

 Longitudinally, these groups of cells are extended for practically the whole 

 length of the anther, except the rare cases where the anther is transversely 

 septate and a longitudinal row of separate pollen-loculi is formed. In some 

 of these cases, the archesporium in each of the loculi may be reduced to one 

 cell, e.g., in Mimosaceae. 



The archesporial cells enlarge radially and divide parallel to the sur- 

 face of the anther, to form an outer layer called the primary parietal layer 

 and an inner layer which is the primary sporogenous layer (Fig. 1265). 

 The cells of the primary parietal layer divide again, sometimes several 

 times, forming radial files of cells of variable depth, which also divide anti- 

 clinally as the anther enlarges. There are usually from three to five layers 

 formed, but the number is not constant, even in one anther. The innermost 

 layer usually contributes to the tapetum, and the outermost layer forms 

 part, or sometimes the whole, of the endothecium of the anther. The middle 

 layers may become disorganized or may persist to form the remainder of 

 the endothecium. 



The primary sporogenous cells may be transformed directly into pollen 

 mother cells, or they may divide once or several times before being so 

 transformed. 



The formation of the pollen mother cells is generally regarded as being 

 the end of the sporophyte stage in this direction and it is often marked by a 

 resting period. Many spring flowers are well advanced in development 

 before the preceding winter begins and the anthers may be found to be in 

 the mother cell stage by September or October. If the buds are formed 



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