Closed Nectar-cavities. 99 



variety of form as to render it a difficult task to give 

 a general view of them. 



For our purpose it will perhaps be best to divide 

 them into two groups, one containing those structures 

 which completely cover in and enclose the nectar, the 

 other containing those in which the closure is incom- 

 plete, a narrow orifice being left through which animals 

 can insert their instruments of suction. 



In the former group, then, we have completely closed 

 cavities, to which insects have no means of access 

 except by lifting up or pushing asunder the over-arch- 

 ing, or dilated and closely approximated, parts. Not- 

 withstanding the elasticity and flexibility of the parts, 

 such a separation of them always implies a vigorous 

 push, and requires a certain amount of strength in the 

 animal that effects it ; so that the smaller and weaker 

 kinds are as a rule effectually excluded from these 

 perfectly enclosed cavities. Nor is it hard to recog- 

 nise the close relation that exists between the strength 

 or size of the visiting insect and the mechanical 

 arrangements of the visited flower j and it may be laid 

 down as a general rule that whenever animals are of 

 such small dimensions as not necessarily to come into 

 contact with the pollen and the stigma of a given 

 flower when visiting it, they will also be without the 

 strength required to push asunder the parts which 

 form its nectar-cavity. 



The walls which enclose the nectar-cavity may be 



