112 



KNOWLEDGE 



[.Ti-NE 1, 1892. 



as an inducement, while the scent and brilliant colours 

 serve to attract the attention of the honey-gatherers. 

 The researches of the late Charles Darwin demonstrated 

 the importance of cross-fertilization in the vegetable 

 kingdom. ^'ery many flowers are quite sterile with their 

 own pollen ; in other cases, altliough the flower has the 

 capacity of self-fertilization, the resulting seeds are (if very 

 inferior quality compared with those obtained as the 

 result of cross-fertiHzation. As carriers of pollen, then, 

 insects perform an essential service to plants, and it is 

 in order to secure their services that flowers are brightly 

 coloured. 



For the variety of colour observed among flowers there 

 appear to be two principal reasons. A Httle reflection will 

 show that, since flowers are so dependent on insects for the 

 conveyance of their pollen, it must be to the advantage of 

 each species of plant to possess flowers distinctively coloured 

 and capable of being easily recognised by honey-seeldng 

 insects. A bee does not visit all flowers indiscriminately ; 

 it would be greatly to the flowers' disadvantage if it did. 

 In the course of a single journey the bee for the most part 

 restricts itself to the flowers of one species, and has been 

 known to visit as many as thirty dead-nettles in succession, 

 passing over all otlier flowers. Time is saved by this 

 method, for by keeping to one kmd of flower at a time 

 the insect becomes familiar with its outs and ins, and the 

 practice thus acquired enables it to overtake a larger 

 number of blossoms than it could if it did not observe this 

 rule. This constancy in visiting the same kind of flower 

 is of great importance to plants, since it insures that the 

 pollen will be conveyed to a flower of the same species as 

 that from which it came. But if all flowers were coloured 

 and perfumed alike, the winged botanist could not identify 

 the species ; the pollen would be constantly transferred to 

 the stigmas of the wrong flowers, where it would be useless, 

 and so the work of cross-fertilization would be seriously 

 impeded. 



A second cause contributing to the variety observed 

 among flowers is the desirability of attracting special kinds 

 of insects. As we have just seen, an insect does not visit 

 all kinds of flowers indiscriminately ; neither, on the other 

 hand, does a flower attract indiscriminately all kinds of 

 insects. Not only are injurious and improfitable visitors 

 excluded, but the more specialized insects are in greatest 

 demand. Partiality for particular insects is shown both by 

 the shapes and colouring of flowers. Open shallow flowers, 

 with exposed honey accessible to almost all insects, have, 

 as their most frequent visitors, short-lipped flies and 

 beetles. Many blossoms, again, have become specially 

 adapted to bees. Their honey is placed beyond the reach 

 of short-lipped flies, and requires the .slender proboscis of a 

 bee or butterfly for its extraction. Honeysuckle. Habeuaria, 

 Plumbago, Phlox, and Narcissus illustrate a third type, 

 with flower-tubes so narrow and deep that their nectar 

 is quite inaccessible even to bees, and is reserved entirely 

 for moths and butterflies, which possess an extremely long 

 and thin proboscis. There is a corresponding adaptation 

 in the colours ; the gay tints of the buttercup, poppy, ; 

 and rose appear to have special attractions for beetles ; 

 bees show a decided preference for blue, and this colour 

 predominates in flowers whose shapes are adapted to their 

 visits. Deep tubular flowers specialized for Lepidoptera, 

 fall into two divisions, according as they solicit the 

 attentions of diurnal butterflies or nocturnal moths. Red 

 and purple are the favourite colours of the former, while 

 nocturnal moths show a preference for white and pale 

 flowers. Thus, the carnation and campion (i.i/chnis 

 •liiiniii) which open by day, have dark tints in comparison 

 with Li/r/iiiis rcsju'rtiiia, which unfolds its petals towards 



evening. Almost scentless by day, this white nocturnal 

 flower difluses a delicious fragrance in the twilight. 

 The evening primrose ((Enothera), which however, has 

 yellow petals, is another example of this class. But 

 the most remarkable plant of this type is the night- 

 flowering stock (Cereus). Its pale blossoms open about 

 seven in the evening, emit pufl:s of odour from time to 

 time, and close up again towards midnight ; by morning 

 the flowers are withered. It is impossible to doubt that 

 we have in this instance a flower specialized for the visits 

 of nocturnal moths. The reason why nocturnal flowers, 

 like the honeysuckle and evening campion, have pale- 

 coloured petals, is not far to seek. These pale hues 

 can be much more easily distinguished at night than 

 the red and purple of Dianthus or Githago. Among 

 lilies, both diurnal and nocturnal flowers occur, and 

 clearly indicate by their colours to which section of the 

 Lepidoptera they are adapted. The Turk's-cap lily, with 

 its perianth of fiery scarlet, is a characteristic example of a 

 diurnal flower adapted to butterflies which wander abroad 

 in day-time. On the other hand, Lilium Martagon and 

 L. candidum with their white bells are nocturnal lilies 

 fertilized by night-loving moths. 



Two flowers, unlike in their colouring, can hardly be 

 equally attractive to the same visitors, even if they grow 

 together on the same plant, as is the case in Arnebia ; the 

 presumption, therefore, is that its spotted and pale blos- 

 soms are adapted for different insects, lloreover, the 

 stronger colom-s of the younger flowers correspond with 

 those of the day-blooming class, while the paler tints of 

 those in the second stage will render them more attractive 

 to nocturnal moths ; and this view is strongly confirmed 

 by the fact that night-blooming flowers are never varie- 

 gated, but have their petals uniformly devoid of markings. 

 By night the dark spots tend, in this instance, to conceal 

 the blossoms so much, that, if these are to be converted 

 into nocturnal flowers, the removal of the spots is abso- 

 lutely necessary. We may therefore conclude with toler- 

 able certainty that the flowers of Arnebia in their first 

 stage are adapted to bees and diurnal Lepidoptera, while 

 in their second condition they array themselves in paler 

 hues to attract nocturnal moths. By the colour-change, 

 in this instance, a diurnal is converted mto a nocturnal 

 flower, and one advantage thereby gained is that the blos- 

 soms appeal to a larger class of fertilizing agents. The 

 more restricted the circle of visitors on which any plant 

 depends the greater the risk, in the event of insects being 

 scarce, of its flowers remaining unfertilized and perishing. 

 Here it would seem that Nature proceeds on the same 

 principle as a fisherman in changing his bait. Like some 

 other variable blossoms, Arnebia is in the advantageous 

 position of carrying two strings to her bow. 

 (To III' (■(tntinued.) 



Utttcrs. 



[The Editor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



SWIFT'S COMET. 

 To ttie Editor of Knowledge. 



Dear Sir, — We were unable, owing to cloudy weather, 

 to see Swift's comet until the morning of March 11th ; it 

 was then very hazy, and clouds were passing. With the 

 large telescope, only a faint elongaoion on one side indicated 

 a tail ; no sign of rays or other peculiarities could be seen. 

 The star camera was turned to it and a photograph taken 

 with 110 minutes' exposure. When developed, this showed 



