THE PRINCIPLE OF COHESION. 63 



margins show every degree of union from a mere contact 

 withont real cohesion, thence, cohesion by contact, to a 

 solid central axial structure formed by congenital cohesion. 

 Lastly, the ovary may be one-chambered, with a free-central 

 placenta, as in Caryophyllece and Primzdacece ; or with one or 

 more ovules attached at the base, as in Bumex, Composites, 

 Graminece, etc. It is these latter kinds especially which 

 have given rise to mucli discussion as to the real nature of 

 the placentas, and as to how far the axis enters into tbeir 

 construction. To ascertain this latter point, a study of the 

 distribution and structure of the fibro-vascular cords of 

 the axis and of the carpels would seem to afford the most 

 promising clue to the interpretation. 



It has been already mentioned that the dorsal cords of 

 carpels generally arise by lateral division from those of the 

 sepals or petals; and then the carpels will be superposed to 

 the one or the other of these organs respectively ; * or, a group 

 may emerge from the axial cylinder in a horse-shoe form, as 

 seen in section ; the outermost cord becoming the dorsal- 

 carpellary, and the ends of the curve the marginal. This is 

 the case, for example, in Cyclamen. 



The point, then, at which the carpellary cords branch off 

 from a common stem in the first case may be regarded as 

 marking the termination of their axial character ; and in the 

 latter case, at the separation of parts of the " horse-shoes " 

 to form groups of threes. With regard to those cords which 

 become marginal and placentary, it is important to notice 

 the position of their spiral vessels. f If they are situated on 

 the side of the cord nearest to the medulla, the cord may 



* See pp. 23, 24, and 42, 43, 44. 



t The cords are, of course, reduced to vessels and soft bast only, the 

 former being mostly spiral, but occasionally becoming more or less 

 reticulated. I shall adopt the usual word Trachea;. 



