536 THE FERTILISATION OF FLOWERS. [PART III. 
flowers, they are applied directly to the stigma. Each pollinium 
consists of a great number of packets of coherent pollen-grains ; 
the packets are bound together by delicate elastic filaments to - 
form an oval pollen-mass. When this is pressed against the very 
adhesive stigma and again withdrawn, all the packets that have 
come in immediate contact with the stigmatic surface remain ad- 
hering to it, for the elastic filaments break before the attachment 
of the packets to the stigma gives way. By this means, an insect 
flying from flower to flower effects cross-fertilisation regularly. 
This view of the flower of Orchis was propounded by Darwin 
before he or any one else had succeeded in observing insects en- 
gaged in the work. Every detail in the structure of the flower 
seemed to be satisfactorily explained, but the absence of honey in 
the spur remained without explanation. Sprengel, who supposed 
that the pollinia were applied by insects to the stigma of the same 
flower (702, p. 401), suggested that the insect-visitor came in 
search of honey, and on finding none, passed on to some other 
kind of flower. But it was essential for Darwin’s theory that 
each insect-visitor should visit a number of the flowers in succes- 
sion; and Darwin suggested that possibly the insects pierced the 
delicate tissue of the spur and sucked the included fluid. My own 
direct observations have confirmed this view, as well as every 
detail of the rest of Darwin’s account. The following description 
of the action of insects on Orchis was published many years ago 
(565) — | 
On May 6, 1869, I and my son Hermann at length succeeded in 
observing humble-bees fertilismg the flowers of Orchis upon 
Stromberg Hill. As we lay upon the turf, which was overgrown 
with Orchis mascula, we saw a humble-bee (apparently Bombus 
terrestris) alight, close beside us, on the base of a spike of that 
plant. It thrust its head into a flower, and drew it out after about 
four seconds with the two pollinia attached to it. It repeated the 
same operation on two more flowers. After withdrawing its head 
from the third, it paused, and tried without success to free itself 
from the pollinia, which were cemented firmly to the front of its 
head. Climbing a little further up the spike it thrust its head 
into a fourth flower. At that moment I tried to catch it in my 
net, but it escaped and flew away. We next saw a Bombus 
hortorum suck three or four flowers on a _ spike of Orchis 
mascula (beginning at the base of the spike), and then proceed 
to another spike, near by. We examined this latter spike, and 
found several of its stigmas smeared with pollen and the loculi of 
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