86o ECOLOGY 



such an instance symbiosis is more obligate for the insect than for the flower. The 

 orchids, as a class, show the most extreme floral specialization, dependence upon 

 some particular insect often being obligate. 



Arum. In Arum, although entirely different structures are involved, there is 

 much to recall the cross pollination of Aristolochia (p. 853). The inflorescences 

 are composed of staminate flowers above and of pistillate flowers below, which are 

 arranged on a club-shaped central axis, the spadix, and enclosed within a large 

 bract, the spathe, which, though enlarged below, is considerably constricted above. 

 At anthesis the flowers give forth a nauseous odor that attracts numerous small 

 flies, whose exit for a time is said to be barred by reflexed hairs between the pistillate 

 and staminate flowers and in the narrow passageway above. The pistillate flowers 

 mature first, and when the staminate flowers mature, the lower ring of hairs dies, 

 permitting the insects to crawl over the stamens, where they become covere'l with 

 pollen. Very soon the upper ring of hairs withers also, permitting escape to the 

 exterior. If the insects visit another inflorescence at once, it is evident that the 

 mature pistillate flowers are likely to be cross-pollinated. Since Arum, as veil as 

 most other aroids with similar features, is monoecious, it is obvious that a highly 

 specialized mechanism of this sort prevents not autogamy, but geitonogamy. Re- 

 cent observations call most of this familiar account in question, especially as to 

 Arum maculatum. In this species it is claimed that the exit for the visiting lies is 

 not barred, since the hairs are not stiff enough to impede the insects and often are 

 not even long enough to fill the passageway. So far as the insects are held in the 

 spathe, it is said to be due to drugging by the plant, and it is claimed tha: there 

 is a sufficient amount of overlapping in the periods of maturity of the staminate and 

 pistillate flowers to result in geitonogamy. Furthermore, there is no adequate proof 

 that the flies which manage to escape visit other inflorescences sufficiently soon to 

 effect cross pollination. 



The fig. The most remarkable of the known cases of cross pollination whose 

 evolutionary development cannot even be imagined, is that of the figs, a group of 

 plants which, like the aroids, are diclinous and commonly placed low in the scale 

 of seed plants. The inflorescence, known as a synconlum, is unique; the numerous 

 flowers line the walls of a chamber (representing the receptacle) and are entirely 

 hidden (figs. 1186, 1187). Entrance to the flowers is possible only through \\ small 

 apical orifice (as in the India-rubber tree, Ficus elastica), which is lined with scales. 

 AH species of figs are diclinous, some having the two kinds of flowers soriewhat 

 indiscriminately mixed; in most species, however, the staminate flowers are oward 

 the top and the pistillate flowers toward the base of the synconium. Some species 

 approach dioecism, certain trees having pistillate and others monoecious synconia. 

 The fig of commerce (Ficus Carlca) is essentially dioecious, the pistillate :lowers 

 of apparently monoecious synconia being sterile; in rare cases, however, the stam- 

 inate trees bear some pistillate flowers and even ripen seeds. 



The commercial fig is pollinated by a small wasp, Blastophaga glossorum. whose 

 life cycle is most extraordinary. The females force their way through the synconial 

 orifice, and this is so difficult of accomplishment that usually they lose their wings 

 in the process; after laying their eggs within the young ovules, they die within the 

 growing fig. Those females that chance to enter the pistillate synconia (which 

 become the figs of commerce) have no progeny, since the flowers have styles of such 



