350 Transactions . — Botany. 



observation 



soon shows this to be a fact. On a warm sunny day it is almost impossible to 



watch a bed of this Orchid for any length of time without seeing numerous 



Diptera flitting from flower to flower, busily engaged in robbing them of 

 their sweets. 



If we now call to mind the manner in which the column arches over the 

 lip, we can easily see that an insect crawling into the flower to get at the 

 supply of nectar can hardly avoid touching one of the points of the rostellum, 

 ranging almost directly over it ; if it did so, the delicate exterior membrane 

 would be at once ruptured, and the viscid mass firmly glued to the insect's 

 back. Thus, on withdrawing from the flower, the visitor would carry away 

 with it not only the portion of the rostellum which it had touched, but also 

 the attached pair of pollinia. These (from each pollinium being nearly 

 subdivided into two) would form four little projections standing rigidly erect 

 on the back of the insect; and consequently, when conveyed to another 

 flower, can hardly fail to strike the overhanging stigmatic chamber, which is 



the insect, thus ensuring the fertilisation of the flower. 



As I have several times seen insects remove the pollinia, and on one 

 occasion also seen a pollen-mass left on the stigma, there can be little doubt 

 that fertilisation is conducted on this plan. That insect aid is absolutely 

 required is proved by the fact that the pollinia remain in their cells, and 

 never reach the stigma, when the plant is covered up or allowed to expand 

 its flowers in a room. But, under natural conditions, the flowers are so 

 frequently visited that the pollinia are generally removed directly after the 

 opening of the blossoms ; while the large proportion of capsules produced is 

 good evidence of the completeness with which the visitors perform their 

 duties. Out of eighty-seven flowers, borne on fourteen plants, no 1 s than 

 seventy-one matured capsules, and of those that had failed to do so, many were 

 imperfect^ ones situated at the summit of the panicle, and probably incapable 

 of producing seed. Another set, from a different locality, had borne forty-seven 

 flowers, of which no less than forty-four had ripened capsules. 



The fact that almost every perfect flower produces a capsule, is in 

 remarkable contrast to what occurs in several other genera of our Orchids. 



Pterostylis is fertilised on a plan much more complex, and the 

 co-adaptation of the various parts of the flower is so complete that almost 



instance 



not the case in Acianthus. Yet, from some reason, probably from the want 

 of sufficient attraction, the flowers are comparatively seldom visited, and 

 consequently few capsules produced. In my account of the fertilisation of 

 this genus, (Transactions of the N. Z. Institute, Vol. V, p. 356.) I have 



