THE PHYSIOLOGY OF PLANTS 



§ 42. How are single and how are double flowers developed? 



If we examine the most perfect type of flower that is, one 

 provided with a calyx and corolla, we find the modified shoot, 

 which we term a flower, begins its development as a slender 

 meristematic papilla, at the base of which a number of small 

 ring-like outgrowths appear. The papilla, which first makes 

 its appearance, is the axis of the flower, and the first out- 

 growths from its young cortex {pcriUem) are the calyx 

 leaves. The first leaf rudiments grow more rapidly on the 

 outside than on the inside, and thus form concave structures 

 which arch over the apex of the axis and protect it. Pro- 

 tected by these leaves, another series of outgrowths appear 

 farther up the axis, agreeing in arrangement and number with 

 the petals. They are indeed the rudiments of the petals, but 

 they behave differently from the sepals in their development, 

 remaining rudimentary and scale-like for a long time, even 

 when the new outgrowths arising above them have been trans- 

 formed into stamens and carpels. It is only when the flower 

 approaches the period of expansion that the petals grow very 

 rapidly, become coloured, and by their increase in size burst 

 open the calyx. 



According to this representation, therefore, the flower must 

 be conceived of as a shoot at the base of which the sepals are 

 formed, and which produces nearer its apex the other floral 

 leaves, the petals and stamens, while quite at the apex the 

 carpellary leaves arise and form the pistil. If we conceive the 

 axis which bears these floral organs to be made of a plastic 

 substance such as clay or putty, and imagine the apex of 

 the axis introverted, so as to form a cup-like structure, we 

 should find the pistil in the centre of this hollow receptacle, 

 and the other leaves higher up on the margin. This is the 

 case in the so-called perigynous and epigynous flowers, of 

 which Fig. 30 represents one in longitudinal section. It is a 

 young apple-blossom. 



In this flower the actual apex of the shoot, which in the 

 previously described (hypogynous) flower occupied the summit 

 of the flower, is found at the base of the cup, and the cells 



