Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/256

so many groups of spiral vessels, in every case traced down to one of the six large ovarian groups. They alternate in five whorls, as represented; but I have not attempted to give the actual distances at which they stand apart. In order to guide the eye, the three central groups running to the three pistils are connected by a triangle.

Fig. 36.

Section of the Flower of an Orchid.

The little circles show the position of the spiral vessels.

a1 a2.S S. Stigmas; S_r, stigma modified into the rostellum.

a1 a2.A₁. Fertile anther of the outer whorl; A₂ A₃, anthers of the same whorl combined with the lower petal, forming the labellum.

a₁ a₂. Rudimentary anthers of the inner whorl (fertile in Cypripedium), generally forming the clinandrum; a₃, third anther of the same whorl, when present, forming the front of the column.

Five groups of vessels run into the three sepals together with the two upper petals; three enter the