62 



KNOWLEDGE. 



[Mabch 1, 1895. 



others, but immediately flies off to another blossom. The 

 ascending habit is an extension of the same principle. 

 Begining at the bottom flov?er and going regularly upwards, 

 the bee applies to the iutiorescence the same principle 

 which it adopts in the case of a flower having several 

 nectaries, for on coming to a flower with no nectar it learns 

 at once that to visit the flowers above this would be labour 

 in vain. The advantage of the ascending method is that it 

 enables the bee to avoid a great many useless visits to 

 newly-opened honeyless flowers towards the top of the 

 inflorescence. A further advantage of this system is that 

 the bee is informed in the first flower it enters if the spike 

 has already been visited, and if so, it can at once turn its 

 attention elsewhere. Such systematic visitation must, 

 therefore, result in a considerable saving of time. 



The first plant on which I remarked bees proceeding in 

 this manner was the gladiolus. The flowers of this plant 

 are dichogamous, as those blossoms are termed in which 

 the essential organs mature at diflerent times. As is the 

 case with most conspicuous flowers, the dichogamy is 

 protandrous, that is, the stamens of each flower ripen 

 before its stigma. Self-fertilization is improbable, as all, 

 or nearly all, the pollen is shed before the stigmas expand. 

 In their first stage the flowers are functionally male ; in 

 the second they are female. Another diiference between 

 the two stages exemplified in Gladiolus is that the stamens 

 and stigmas change their relative positions. At first the 

 stamens are bent forwards ; the anthers stand in the fair- 

 way to the nectar, and it is impossible for a bee to enter 

 without encountering them and dusting itself with their 

 pollen. Meanwhile the unexpanded stigmas remain in 

 the upper part of the flower, behind the anthers and quite 

 out of the way of visitors. When the flower passes into 

 the second stage the stamens, ha\'ing scattered their 

 pollen, straighten up and occupy the upper portion of the 

 flower, beyond the reach of visitors. The style now bends 

 forward and the stigmas uncmd in the entrance to the 

 flower, so that no bee can enter without touching them. 

 By the time the older or lower flowers have attained the 

 second or female stage, many of the younger ones higher 

 up the spike can only have reached the stage in which the 

 pollen is discharged. Hence, a bee entering the lowest 

 flower pollinates its stigma with pollen brought from 

 another plant, and proceeding upwards comes to younger 

 blossoms, where it is dusted with pollen, which it carries 

 off with it to the next spike, where the same thing is 

 repeated. Were the bee to begin at the top and work 

 downwards, it would waste its own time searching flowers 

 not far enough advanced to yield nectar, and simply 

 transfer the pollen of the upper blossoms to the stigmas of 

 the older flowers farther down the same spike. This, 

 indeed, would be a kind of cross-fertilization, but the 

 experiments of Mr. Darwin have conclusively shown that 

 the intercrossing of flowers belonging to the same individual 

 plant yields results little, if at all, better than self-fertili- 

 zation. So numerous are the provisions in flowers for 

 preventing self-fertihzation that there is only one possible 

 conclusion, viz. : that the acropetal inflorescence and 

 protandrous flowers constitute a special adaptation to the 

 ascending habit of insects intended to secure the inter- 

 crossing of distinct plants. 



The wood sage, one of our commonest labiates, illustrates 

 even better than the gladiolus this provision for the inter- 

 crossing of different plants. Its flowers, as will be seen from 

 our illustration, are, like those of Tritoma and Gladiolus, 

 protandrous. The stamens having shed their pollen bend 

 backwards over the upper edge, and the stigma which at 

 first is behind the stamens bends forward and occupies the 

 entrance of the flower. The number of flowers on each 



spike is usually much greater than shown in the sketch. 

 They are developed acropetally, and as the bee in visiting 

 them usually adheres to the ascending habit they must, in 

 most cases, be fertilized with pollen from a diflerent plant. 

 The common figwort is visited chiefly by wasps. On 

 this plant I have repeatedly observed a wasp begin at the 

 uppermost flower and work downwards. But this apparent 

 exception only confirms the explanation just given, for the 

 flowers of the figwort are protogynous, not protandrous, 

 the stigmas being in advance of the stamens, and although 

 the inflorescence develops somewhat differently the 

 arrangement here also leads to the intercrossing of flowers 

 belonging to different plants. 



Simple flowers have the honey exposed and easily 

 accessible to all insects, but in more highly developed 

 blossoms it is often so completely concealed that only 

 intelligent visitors can discover it. Sprengel long ago 

 noticed the stupid manner in which flies behave when 

 searching for nectar. One of the carrion flies constantly 

 misses its aim when trying to insert its proboscis into the 



flowers of 

 bistort. 

 Beetles are 

 ,; equally 



,^^ J'" awkward 



^^A .-'iT and make 



many in- 

 eflectual at- 

 tempts to 

 reach the 

 honey of 

 Erodium. 

 Such stu- 

 pidity is, 

 however, 

 characteris- 

 tic only of 

 short -lipped 

 species, for 

 it has been 

 shown that 

 structural 

 adaptation 

 and intelli- 

 gence ad- 

 vance to- 

 g e t h e r . 



Among flower-haunting insects a long proboscis is indicative 

 of a high grade of intelhgence. Butterflies are especially 

 deft, thrusting their proboscis into flowers and withdrawing 

 it with astonishing swiftness and precision. The dexterity 

 of the more specialized bees is almost equally great, 

 though even bees may be seen sometimes fumbling about 

 in vain for the entrance to the nectary on flowers with 

 which they are not familiar. They soon recognize the 

 value of the guiding lines on petals, and, following them, 

 overtake more work. In this way they are induced to 

 enter flowers in the manner most calculated to efl'ect cross- 

 fertilization. Less intelligent guests, unable to interpret 

 the honey -guides, might remove the nectar without con- 

 ferring any benefit on the flowers. Changeable flowers 

 like Eibes indicate to intelligent visitors if they have 

 already been visited, and divert the less iutelhgent from 

 the unvisited blossoms still containing nectar. Sharp- 

 witted guests alone are likely to enjoy the honey in th3 

 deceptive flowers of Parnassia. The blaeberry, again, 

 illustrates a class rich in nectar, but rather obscurely 

 coloured and frequented by bees sufliciently wise not to be 

 misled by appearances. Flowers are thus adapted not only 



The Wood Sage {Teucrium scorodonia). 



1. Young upper flower in male stage. 2. Older 

 lower flower in female stage, st Stigma. 



