98 THE STRUCTURE OF ECONOMIC PLANTS 



undergoes the reduction divisions which result in a tetrad of 

 microspores (young pollen grains). 



Coincident with the formation of the microspores, digestion of 

 the non-sporogenous tissues leads to the formation of the locules 

 of the four sporangia; and, frequently, the walls between adjacent 

 sporangial cavities disintegrate so that the mature anther contains 

 two locules or pollen sacs. Dehiscence of the anther occurs in ex- 

 tremely varied ways. In some cases, a longitudinal splitting occurs 

 along a definite stomtum. In others, there is a small apical fissure, 

 a terminal pore, or an irregular breaking of the tissues. The wall 

 tissues of the pollen sacs are frequently hygroscopic; and when 

 they become moist, the endothecial cells may close the stomia 

 again, in this manner covering any pollen which has not been shed. 



At the time of shedding, the pollen grains have usually passed 

 beyond the microspore stage, their nuclei having divided to form a 

 ttihe and generative nucleus. In some instances, the latter may have 

 again divided so that at the time of shedding, the pollen grains are 

 trinucleate. Thus the mature pollen grain may be regarded as a 

 micro gametophyte, which consists either of a tube and generative 

 cell, or a tube cell and two micro gametes. 



Development of the Carpel. — The carpels may develop singly 

 as in some of the Rosaceae and Leguminosae, or several of them 

 may develop conjointly or undiverged so that the resultant pistil 

 is compound (syncarpy). When a single simple carpel develops, 

 the primordium of the carpel, or megasporophyll, generally arises as a 

 more or less open structure whose margins, because of differential 

 growth, eventually meet, leaving a slight furrow or suture at their 

 junction which in some instances is complete. The basal portion 

 of the carpel constitutes the ovary, the terminal portion of the 

 primordium elongates to a greater or lesser degree to form the style 

 with the stigma at its tip or along its apical margins. The adaxial 

 suture represents the margin of the sporophyll, its abaxial suture 

 the midrib. There is often a prominent vascular bundle extending 

 the length of the abaxial suture, and one in each of the edges along 

 the adaxial suture. 



Placentation. — The inner ridges of the carpellary margins in 

 the ovarian portion make up the placentae. It is from this region 

 that the ovule or ovules arise. In the simple carpel where the 

 initiation of the ovules is marginal, the placentation is parietal. 

 This type frequently occurs in syncarpous ovaries when the margins 



