284 DEPOSITION OF POLLEN. 



lick up the honey they would be knocked off by the rostellum, and their aim would 

 either fail entirely or be but partially achieved. But, as soon as the little clubs have 

 bent down over the front of the wasp's head, they are planted by this honey-hcking 

 insect exactly on the sticky rectangular stigmatic surface. Each quartet of pollen- 

 cells forms a round or irregularly rectangular ball, and these, connected together by 

 viscous threads, are again grouped so as to form the club-like pollen-mass. When 

 this club is placed on the sticky stigma, all the pollen-quartets which come into 

 contact remain attached, so that when the insect flies away it is much more likely 

 that the sticky threads in the interior of the pollen-mass will be torn than that the 

 pollen adhering to the stigma will be removed again. These two contrivances, so 

 important for the deposition of the pollen on the stigma, viz. the twisting and 

 bending of the originally erect pollen-masses and the tearing of the fine threads 

 which connect the quarte!^ of pollen-cells, occur not only in the Helleborine {Epi- 

 pactis), which has been chosen as an example, but also in many other Orchids 

 which adorn our woods and meadows — especially in the genera Orchis, GymTiadenia, 

 and Habenaria. In the Epipogiwm (see fig. 257, p. 226) the floral contrivances 

 are rather different. Each pollen-mass is chained on one side by the thick strand 

 which leads to the sticky rostellum (fig. 257 ^^). When these masses are torn from 

 their hiding-place by a humble-bee (257 '^) they bend round, and now hang on their 

 supports like two cherries on their stalks. In this way the structure, torn from 

 the anther, becomes somewhat elongated — an important change — since it renders it 

 possible that the clubs should reach the stigma in the next Epipogium flower 

 visited. In this plant the stigma stands above the rostellum, and the pollinia can 

 only be pressed by flying humble-bees against the stigma if they have long stalks. 



Each of these contrivances shows afresh how exact must be the correlation of 

 all the organs which participate in the transference of pollen, and how well they 

 must be regulated if the success of the flower is to be ensured. The alteration of 

 a millimetre in the position of the stigma will prevent the pollen being deposited 

 on the right place and the consequent fertilization. In many cases a still slighter 

 alteration would be hurtful. In some plants only a very limited area of the stigma 

 is able to incite the pollen to emit pollen-tubes. In Asters, as will be shown more 

 in detail presently, it is only a narrow border at the edge of the minute stylar branch, 

 and in many Labiates it is only the tip of the lower branch of the stigma on which 

 pollen can be deposited with successful results. Sarracenia purpurea possesses 

 one of the largest stigmas. It has the form of a sunshade of 3'5 cm. diameter, with 

 five indented lobes round the edge, and the margin of each lobe is furnished with 

 a small tooth on the inside (see fig. 279^) These teeth alone are fitted to receive 

 pollen, and if the term stigma is to be restricted to the tissue on which the pollen 

 can eventually develop and put out pollen-tubes, it will only refer in Sarracenia 

 to these five tiny teeth. The same is true of Physostigma venenosum (see figs. 282 ' 

 and 282 ^) whose bladder-like stylar termination, described as the stigma, is only 

 capable of real pollination over a small part beset with papillae. It should also be 

 noted here that the papillae which are developed on the outer side of the stylar 



