I2I4 A TEXTBOOK OF THEORETICAL BOTANY 



podium, fruits of two different forms are produced at different points in the 

 inflorescence and in Polygonum fruits which are either two-angled or three- 

 angled are formed apparently indiscriminately. The biological reasons for 



Fig. 1 182. — Dimorphic fruits of Dhiiorphotheca. The ray florets produce cyhndrical fruits, 



the disc florets produce flattened fruits. 



these differences are not known, but in cleistogamic flowers (see p. 1355) 

 the fruits are often different from those produced by the flowers of the same 

 plant which are openly pollinated. 



The ontogeny of the carpels from the receptacle is best illustrated by 

 the free carpels of apocarpous flowers. The earliest stage is a hemispherical 

 protuberance closely resembling the primordium of a leaf. The further 

 development in many cases resembles that of a peltate leaf rather than that of 

 a leaf of normal type, though the three modes of growth characteristic of 

 leaf primordia, namely, longitudinal growth, marginal growth and growth in 

 thickness, proceed in the same order. 



Troll has shown that the growth of the carpel in breadth proceeds from 

 sub-marginal cells as in the foliage leaf and that it is only in the stylar region 

 that there is any considerable growth in thickness. The carpellary stalk, 

 when present, has a unifacial structure like that of the petiole of a peltate 

 leaf and he uses this as an argument in support of the peltate character of 

 many carpels, even where, as in some peltate leaves, the subsequent growth 

 is not fully peltate. 



The development of the typically peltate carpel has been compared with 

 that of the abnormal ascidial leaves which occur in many genera, and with 

 that of the pitcher leaf in Sarracenia, which is a very pronounced ascidial 

 leaf. A depression appears at the apex of the primordium, and the opposite 

 sides of the depression develop unequally. The abaxial lip grows rapidly in 

 length, and forms the back or dorsum of the carpel, in which the dorsal 

 bundle lies and which is produced upward into the style. The adaxial lip 

 grows more slowly and forms a sort of ridge, called by Goebel the " sill " 

 (Sohle)* of the carpel (Fig. 1 183). If it extends itself upwards it forms the 

 two margins of the carpel, which are thus united from the start. Where the 

 ovules are reduced to one, this usually arises from the sill and is median in 



* The word Sohle was used by Celakovsky to denote a hypothetical phnth on which the 

 carpel was supposed to be based, but in the sense in which it is applied to peltate carpels by 

 von Goebel, it is better translated by " sill " than by " sole ". 



