FLOWER STRUCTURE 195 



central fibro- vascular bundle of each filament. The 

 fibro-vascular elements consist of spiral or annular 

 vessels with practically no fibrous tissue. The pollen 

 (Plate 64) is the most important histological element 

 of the stamen tissues, for in many instances the grains 

 present characters of great importance to the analyst. 

 Pollen grains, although uniform in a given plant, 

 show great variation in size, form and surface mark- 

 ings. They may be spherical, ellipsoidal, triangulate 

 or polygonal hi form. Pores, grooves and elevations 

 may be apparent on the outer surface. In many 

 instances the outer surface shows numerous spiny 

 projections which are probably for the purpose of 

 firmly fixing the grain upon the stigmatic surface. 

 Grains possessing these spines or projections are 

 termed spinose hi contradistinction to the smooth- 

 surfaced grains. 



Pistil Tissues. The pistils collectively constitute 

 the gyncecium or female reproductive structures hi 

 the flower. Each pistil consists of stigma, style and 

 ovary. The stigma is the uppermost portion of the 

 pistil and is connected with the ovary by the style. 

 The histological elements present in the pistil include 

 epidermal tissues, parenchyma, fibro-vascular elements 

 and ovules. The epidermal tissues of the stigma 

 are of two distinct varieties. The upper surface of 

 the stigma or the surface designed as a resting place 

 for the pollen, is usually roughened either by papillae 

 (Plate 64), or by a peculiar palisade formation of the 

 epidermal cells. In other instances the stigmatic sur- 

 face is smooth, but secretes viscid liquids which hold 

 the pollen when the latter comes into contact with it. 

 The style and ovary are covered with a thin epidermal 



