396 



ANGIOSPERMAEMONOCOTYLEDONES 



provided with a nectar-guide, forms a convenient alighting-platform for insects. 

 Its cylindrico-conical spur does not secrete nectar, but affords a liquid^ enclosed 

 in cellular tissue to visitors, which extract it by boring. The two anther-lobes 

 are firmly united with the column, each possessing a special adhesive disk covered 

 by a bursicula. There is a rostellum between the two anther-lobes, which presses 

 them apart; they dehisce in the front by means of a longitudinal slit from top to 

 bottom, the front of the poUinia being thus exposed; these contract into a sort 

 of stalk towards the base and are here united to the epidermis of the bursicula. 



Fig. 364. A. Flower of Orchis maculata, L. (after F. MacLeod). (i, 2, 3) The sepal and upper 

 petals which make up a protective roof. (4, 5) Lateral sepals. (6) Labellum. (6 s) Lateral lobes 

 of do. /;, bursicula; hb, uppermost part of protective roof; sp, entrance to the spur; */, stigma; 

 ss^ open anther loculus with poUinium ; vl, membranous appendage of A. 



B. Pollinium of Orchis tnascula, L. {]. MacLeod after Darwin), a, immediately after withdrawal from 

 the anther (on a pencil) ; b, do., bent forward, after exposure to the air. 



An insect inserting its proboscis into the spur strikes its head against th< 

 bursicula, the epidermis of which is ruptured, so that the two small, circular adhesiv^^ 

 disks at the ends of the pollinia cling to the visitor's head and become firmly fastened^^ 

 to it by the immediate hardening of the viscid substance. On withdrawing its head 

 from the spur the insect thus removes the pollinia ; the pollen-masses are drawn out 

 of their receptacles and harden in the open air, their caudicles bending more and 

 more forward. (This process can easily be imitated by means of a small sharp style 

 or pencil.) The pollinia ultimately turn through an angle of almost 90, so that 

 when the insect visits another flower they strike directly against the stigmatic 

 surface situated below the rostellum in the entrance of the spur. As the 

 adhesiveness of the stigma is greater than that of the pollinia to the insect, they 

 are torn away and left behind when it backs out of the spur. Cross-pollination 

 therefore always takes place when insects visit the flowers, and not only crossing 

 with separate flowers, but also with separate stocks, may be eifected if the insect 

 has visited another plant during the time necessary for the bending forward of 

 the pollinia. Should insect-visits fail no fertilization takes place, because automatic 

 self-pollination is impossible. 



' Cf. the footnote on Leucojum aestivom L. 



