70 THE STRUCTURE OF FLOWERS. 



notwithstanding tlie fact that the tracheao are oriented 

 inwards ; since it is not until they reach the level of the 

 insertion of the ovules that they pass either to the middle 

 or opposite side of the cord. The rest of the carpellary 

 tissues are undifferentiated, as stated above, and it is this very 

 common condition in the case of inferior ovaries that has led 

 botanists to regard the lower parts of the carpels as being of 

 an axial nature and not foliar. 



The Formation of Septa. — With regard to the union of 

 the surfaces of the carpels to form the septa, the rule is for 

 the adjacent epidermides to be altogether wanting ; and, if 

 the median tissue be thick, the walls of two adjacent ovary- 

 cells may be very wide asunder, as in the Ivy. On the other 

 hand, the septa may be reduced to the two epidermal layers 

 alone, and then they are often scarcely coherent at all, as in 

 Balsam and Lemon. 



In some cases, the epidermides are not in contact through- 

 out their entii^e surfaces, and whenever this is the case the 

 characteristic epidermal cells reappear, as in Liliacece and 

 AmaryllidacecB. Similarly, as soon as the carpels of Hellebore 

 become free, the epidermides of the margins appear in their 

 proper character, which now cohere only by contact. It is 

 the same with the axile placentas of the Lily. 



As instances where the axis seems to be more decidedly 

 prolonged up the centre, are Lychnis and allied members of 

 the Silenece. Ph. Van Tieghem has also shown how an axial 

 cylinder ascends up the middle of the flower of Campanula 

 medium for about two- thirds of the height. Thus Fig. 15, a, 

 represents a section of the fluted pedicel ; b shows the lobes 

 isolated, each containing a portion of the fibro-vascular 

 cylinder. In c, the broken central cylinder has again closed 

 up, a section showing a complete cii'cle of an axial character. 

 The triangular basal portions of the ovary-cells have now 



