THE STIGMATIC SURFACE. 283 



centre, in the Prickly Pear (Opuntia; see fig. 279^^) it forms a sinuous fleshy 

 swelling which winds about the end of the style, whilst in the Evening Primrose 

 {CEnothera; fig. 281) it is composed of four fleshy lobes arranged in a cross. It is 

 noticeable that sticky stigmas occur most frequently in plants whose pollen is 

 liberated from the sprinkler-like anthers as flour or dust. Such flowers also, the 

 pollen of which consists of quartets (tetrads) of cells surrounded and entangled in 

 delicate threads, are characterized by very sticky stigmas (cf. fig. 219 ^ p. 101). In 

 most of the plants mentioned above the pollen adheres so firmly to the stigma at 

 the moment of contact that it cannot be removed even by blowing or vigorous 

 shaking. Many of the sticky stigmas remind one of limed twigs, especially as 

 the sticky layer which produces the adhesiveness is exposed to the air and yet does 

 not dry up, but remains sticky and viscous like bird-lime for several days. 



In many instances the stigma does not become sticky until the stigmatic tissue 

 is capable of inciting the pollen-cells which come in contact with it to put out pollen- 

 tubes. The stigma of Cephalaria alpina, one of the Dipsacese, is very remarkable 

 in this respect. Shortly after the corolla has opened, the stigma appears to be 

 completely matured, and as if capable of retaining pollen. But this is not really 

 so. Any pollen placed on it immediately slides ofi" its smooth surface. Not until 

 two days later when the stigmatic tissue has become covered with a delicate layer 

 of sticky fluid, scarcely visible to the eye, is the pollen held fast, and at once puts 

 out pollen-tubes which penetrate into the tissue. But, as in so many other cases, 

 it is impossible to generalize on this point, thus in most Umbelliferse the stigmas 

 are sticky before their tissue is able to influence the pollen in this way. Also, in 

 the flowers of Alliiiim Victorialis, the pollen adheres to the stigmas before these 

 are capable of inciting the emission of pollen-tubes, indeed, at the time of attach- 

 ment the stigmatic papillee are not even developed. The stigmas of Orchids are 

 sticky some time before the ovules are matured. In these cases the sticky layer 

 has to retain the pollen until the changes have been completed in the deeper 

 stigmatic tissue which will stimulate the pollen to put out its tubes. 



It is necessary to give a special description of the manner in which the pollen 

 is deposited on these sticky Orchid stigmas. The stigma of the Helleborine {Epi- 

 pactis latifolia), illustrated in fig. 268, p. 255, has the form of a rectangular table, 

 and is placed opposite the boat-like labellum, which is filled with honey. When a 

 wasp, in licking out the honey, strikes its head against the projecting rostellum at 

 the upper margin of the stigma it adheres for a moment. The two club-shaped masses 

 of pollen which are connected with the rostellum are thus torn out of the loculi of 

 the anthers, and removed by the wasp as it flies away. The wasp now carries the 

 pair of pollen-m.asses on its head as shown in fig. 268 ^. At first these pollinia stand 

 erect, but after a few minutes they alter their position. In consequence of drying, 

 the masses, composed of adhering groups of pollen-cells, twist and become deflected, 

 and now appear as two thick cushions lying on the front of the head (see fig. 268 ''). 

 This bending is absolutely necessary if the pollen is to be brought by the wasp to 

 the sticky stigma of another flower. If the wasp came with erect pollen-masses to 



