90 THE STRUCTURE OF ECONOMIC PLANTS 



Three distinct parts usually comprise the pistil, the basal ovary, 

 the more or less elongated style, and the stigma, which is terminal 

 or lateral in greater or lesser degree. The receptive surface of the 

 stigma, upon which the pollen grains lodge and initiate growth 

 of the pollen tube, may be papillate or plumose and feathery. In 

 many cases, glandular hairs provide a sticky secretion which 

 stimulates growth of the pollen. The entire carpel, or some por- 

 tion of it, constitutes the true fruit, and the seeds are developed 

 and contained within it. In many cases, other structures are 

 associated with it at maturity, and the assemblage is called the 

 fruit. 



Floral Ontogeny. — In the ontogeny of the flower, the primor- 

 dium first appears as a rounded growing point of meristematic 

 tissue. From this, the primordia of the floral parts arise as lateral 

 structures in a spiral or series of cycles and develop much in the 

 manner of foliar primordia, expanding and developing vascular 

 traces. The subsequent development of the floral axis differs 

 from that of a vegetative one in that there is little internodal 

 elongation, so that the floral parts mature in a compact relation- 

 ship rather than being more or less widely separated from one 

 another. 



In flowers that are regarded as relatively primitive, the ori- 

 gin of the floral primordia in an acropetal succession results in 

 the development of the hypogynous flower. (Fig. 3X.) In a cyclic 

 flower of this type, each successive cycle is definitely above and 

 centrad to the next outer cycle; and the level of divergence of the 

 stamens, petals, and sepals is below that of the carpels. 



In many flowers, development involves some degree of zpnal 

 growth. Instead of each part differentiating as a separate entity, 

 there may be an activation of a whole zone of meristematic tissue 

 with varying degrees of subsequent divergence or differentiation of 

 individual members. Where this tendency toward zonal growth 

 is expressed perigynous or epigynous flowers are formed. 



In perigyny, the elongation of the terminal portion of the floral 

 axis which gives rise to the carpels is inhibited, and the outer 

 cycles of the growing point develop as a zone so that the three 

 outer sets of floral parts grow conjointly at their bases and are 

 diverged from the rim of an urn or cup-like outgrowth as separate 

 structures. This makes it appear that the carpels arise from a 

 depression in the torus or receptacle; but it is actually the result 



