362 OUTLINES OF EVOLUTIONARY BIOLOGY 



the conspicuous colour of the sepals and petals, -which serves ns 

 an advertisement ; the formation of the long spur or nectary, with 

 its secreted honey ; the great development of the labellum to 

 serve as a landing place, marked with the "honey-guide " (Sm) 

 to point out the way to the visitor; the secretion of sticky 

 cement by the rostellum ; the formation of an elastic cap to keep 

 the cement from being dried up before it is wanted; the agglu- 

 tination of the pollen into solid masses, which serve to fertilize 

 a large number of flowers in succession, losing a few grains at 

 each contact ; the remarkable form of these pollinia, with their 

 adhesive discs and their peculiar relation to the sticky substance 

 on the rostellum : and, lastly, the wonderful mechanism by means 

 of which the polknia become bent downwards after their removal, 

 so as exactly to adjust them for c&ntact with the stigma of 

 another flower ! 



In some species of plants certain parts of the flower are highly 

 sensitive and respond to the" slightest touch by a sudden and 

 vigorous movement, going off, so to speak, like a spring trap or 

 a hair trigger. This is well exemplified in a New Zealand orchid, 

 Ptcrostylis trullifolia, as described by Mr. Cheeseman. In this 

 flower the lower petal, or labellum, is highly sensitive, and when 

 an insect alights upon it springs up and imprisons the visitor 

 in a cage. The parts of the flower are so arranged that the 

 insect can only escape by crawling through a narrow passage, 

 and in such a manner that it must carry away the pollinia and 

 also leave on the stigma some of any pollen which it may chance 

 to bring with it. 



A still more remarkable orchid, Catasetum, actually throws the 

 pollinia at its insect visitors as soon as they touch the flower, 

 and the tiny projectiles attach themselves to the intruder's head. 

 Darwin has described how, on one occasion, when he experi- 

 mentally irritated this flower, the pollinium was thrown for a 

 distance of nearly three feet, when it stuck on to a window 

 pane. 



The presence of irritable structures which aid in cross- 

 *fertilization by insect agency is, however, by no means confined 

 to orchids. Ca-udollea (Stylidwm) gramimfoUa is a common 

 Australian wild flower belonging to a totally different order, the 

 Candolleacese. It is found growing on open heaths and sends 

 up tall stems from the midst of a tuft of long, grass-like leaves, 

 each stem bearing a large number of rather small pink flowers 



