2i6 THE PHYSIOLOGY OF PLANTS 



together. Of the three portions of the pistil, the lowest one, 

 the ovary, is the most important, as it contains the ovules with 

 their egg-cells. The stigma, which in some plants is very 

 extensive (feathery in Grasses, disk-shaped in the Poppy), may 

 be looked upon as an organ for catching the pollen, while the 

 style conducts the pollen tubes, which grow out from the 

 pollen grains, down to the ovules. In many cases there is no 

 style formed, and the stigmatic surface is directly on the top 

 of the ovary. 



The sexual organs are, in most flowers, surrounded by the 

 leaves of the floral envelopes, which are either arranged in 

 several whorls or in a spiral manner. In complete flowers 

 the floral envelopes are sharply differentiated by form and 

 colour into two whorls of leaves, the inner one of which, 

 the eopolla, is built up of delicate cells, filled with a colourless 

 or a brilliantly coloured cell sap. The epidermal cells of these 

 leaves are often drawn out into papillae, and give the surface 

 of the petals a velvety appearance. The outermost whorl of 

 leaves, the calyx, is generally green, and resembles greatly in 

 texture, and often also in shape, the assimilating leaves. 



If we cut through a flower longitudinally, we see that the 

 several whorls of leaves appear to spring from one point. 

 In reality they are, however, separated by short but distinct 

 internodes of the axis. 



Each flowep represents, therefore, a reduced shoot with several 

 whorls of leaves, each of which is modified for a special func- 

 tion, but which, under certain conditions of nutrition, exhibit 

 the tendency to assume the nature of a green assimilating 

 leaf. 



Such a retrogression of the flowering axis to a leafy shoot 

 we term proliferation. 



The flowers occurring in some of the natural orders, how- 

 ever, diverge often very considerably from the typical flower 

 described above. One of the most frequent modifications 

 results from the disappearance of the differentiation of calyx 

 and corolla. The floral envelopes are completely alike, and 

 either resemble a corolla (Tulips and other Liliaceous plants), 

 or are green and have the appearance of a calyx {Juncacece 

 and Cupulifercv). In such cases we speak of a perianth. 



