Natural Order Cucurbitaceae. 263 



" 359. Where the margms of the folded leaf, out of which 

 the carpel is formed, meet and unite, a copious development 

 of cellular tissue takes place, forming what is called the pla- 

 centa. 



"360. Eveiy placenta is therefore composed of two parts, 

 one of which belongs to one margin of the carpel, and one to 

 the other. 



"361. As the carpels are modified leaves, they necessarily 

 obey the laws of arrangement of leaves, and are therefore de- 

 veloped round a common axis. 



"362. And as they are leaves folded inwards, their margins 

 are necessarily turned towards the axis. The placentae, there- 

 fore, being formed by the union of those margins, will be in- 

 variably next the axis." 



From this we learn, in few words, that the carpellary leaf is 

 always so folded that its midrib is towards the circumference, 

 or forms the dorsum of the cell or carpel, while the placenti- 

 ferous margins are placed in the axis ; that the difference be- 

 tween a one-celled and many-celled fruit merely consists in 

 the placentiferous margins of the carpellary leaves of the 

 former not extending inwards to the axis, but stopping in the 

 circumference and bearing their ovules attached to the walls 

 of the cell — hence parietal. This position of the carpellary 

 leaf is so constant, that the possibility of an inversion of this 

 order of things in a pepo seems never to have entered into 

 the calculations of any one of the numerous botanists who 

 have given their attention to the investigation of the structure 

 of this curious fruit ; and yet such is simply the case. In a 

 pepo the normal position of the midrib of the carpellary leaf 

 is reversed, that is, is placed in the axis, and the placentife- 

 rous margins towards the circumference. That such is actually 

 the case requires no argument to prove ; we have only to cut 

 the ovary of any true cucurbitaceous plant to be made sen- 

 sible, at a glance, that it is so ; though I confess that in none 

 have I seen it so clearly made out as in Coccinia indica, owing 

 to the carpels of that species remaining distinct, merely held 

 together, not as usual by cohesion between the respective car- 

 pels, but by the tube of the calyx in which they are enclosed. 

 Did I wish to illustrate the theory by means of a diagram, I 

 could not devise one more perfect than a simple section of the 

 ovary of that plant, merely extending the natural divisions, 

 by dividing the calyx, so as to allow each of the carpels to be 

 slightly separated in the representation, to facilitate the de- 

 monstration. This, however, is I think even unnecessary, for 

 with the clue to the true structure which this species fur- 

 nishes, there can no longer be any difficulty in understand- 



