252 GRADATION OF ORGANS. Ciiap. IX. 



cases in which the upper surface of the rostellum is 

 viscid, as in Malaxis and some Epidendrums, and 

 dimply adheres to the pollen-masses ; for these cases 

 present no difficulty. But I refer to the so-called con- 

 genital attachment of the poUinia by their caudicles 

 to the rostellum or viscid disc. It is not, however, 

 strictly correct to speak of congenital attachment, for 

 the polKnia are invariably free at an early period, and 

 become attached either earlier or later in different 

 Orchids. No actual gradation is at present known in 

 the process of attachment; but it can be shown to 

 depend on very simple conditions and changes. In 

 the Epidendrete the poUinia consist of a ball of waxy 

 pollen, with a long caudicle (formed of elastic threads 

 with adherent pollen-grains), which never becomes 

 spontaneously attached to the rostellum. In some of 

 the VandeiB, as in Gymbidium giganteum, on the other 

 hand, the caudicles are congenitally (in the above 

 sense) attached to the pollen-masses, but their struc- 

 ture is the same as in the Epidendrese, with the sole 

 difference, that the extremities of the elastic threads 

 adhere to, instead of merely lying on, the upper lip of 

 the rostellum. 



In a form allied to Cymbidium, namely, Oncidium 

 unguiculatu/m, I studied the development of the cau- 

 dicles. At an early period the pollen-masses are 

 enclosed in membranous cases, which soon rupture at 

 one point. At this early period, a layer of rather 

 large cells, including remarkably opaque matter, may 

 be detected within the cleft of each pollen-mass. This 

 matter can be traced as it gradually changes into a 

 translucent substance which forms the threads of the 

 caudicles. As the change progresses, the cells them- 

 selves disappear. Finally the threads at one end ad- 

 here to the waxy pollen-masses, aud at the other end 



