216 



KNOWLEDGE 



[November 2, 1891. 



seed of which has a marked resemblance to the larva of an 

 insect. The black shining seeds of a large number of 

 plants, of folutiiliiiie among others, might easily be mis- 

 taken for minute beetles, and the similarity may be of 

 advantage in the manner indicated. 



The fertilization of Howers and the dispersion of seeds 

 are, of course, services unconsciously rendered so far as the 

 insects and birds are concei-ned. There is the exceptional 

 case of the Yucca moth, whicli deliberately pollinates the 

 flowers on the seeds of which its larv;e feed. To this there 

 would appear to l)e a curious parallel ; for it is alleged that 

 in Ciuatemala a bird has the remarkable habit of picking 

 holes in the bark of a certain tree, and depositing therein 

 the seeds of a parasitic plant on the berries of which it 

 feeds. 



Dispersion through tJie agency of birds is attended 

 with certain advantages. The necessity for dispersion 

 appears to be common to all plants ; but this mode affords 

 facilities for the transport of seeds across mountain ranges 

 and arms of the sea, such as would prove effectual barriers 

 to wind-borne seeds unless those of infinitesimally small 

 dimensions. Birds are more likely to dehver the seeds in 

 locahties corresponding to the habitats of the plants. 

 The maceration of the seeds in the bird's stomach in some 

 cases appears to facilitate germination. This mode of 

 dispersion is also preferable to wind agency where the 

 number of inchviduals composing the plant species is 

 limited, since few^er seeds will be lost or destroyed. 



It is not very easy to account for the origin of these 

 special pro^■isions favouring dispersion by birds. The 

 colours and sweet tastes of fruits as well as certain pro- 

 perties of the seeds might, no doubt, be referred to 

 variation and natural selection. Possibly these may have 

 arisen very much in the way that Darwin supposed the 

 adaptation of flowers to insects to have been brought 

 about. The poUen of primitive wind-fertilized flowers 

 might, he thought, at first induce insects to visit them. 

 The presence of nectar he explained as due in the first 

 instance to the accidental appearance within the flower I 

 whorls of a sweet secretion occasionally exuded from ! 

 various parts of plants, such as from the leaves of the 

 common laurel and on the stipules of some of the Legu- 

 miuosffi. Flowers which oflered such attraction would be j 

 most frequented, and from being oftener crossed would 

 give rise to more numerous and more \'igorous seeds. 

 Their descendants would inherit these peculiarities in a 

 still higher degree. Colour and perfume would now come 

 to be of service as additional attractions, the deepening of 

 the flower-tubes in order to reserve the nectar for special 

 visitors would follow, while corresponding modifications in 

 the proboscis and other organs of those insects most 

 dependent on the flowers might be expected to take place. 

 In this way Darwin conceived it possible that in process 

 of time the innumerable and wonderful contrivances for 

 eflecting the cross-fertilization of flowers through insect 

 agency might have been called into existence. More 

 recently Henslow has endeavoured to account for such 

 peculiarities in flowers as the colour of the petals, their 

 markings, the secretion of nectar, irritability of certain 

 parts, coronas, spurs, &c., on the theory that they have 

 been called forth as the result of a localized flow of nutri- 

 ment induced through irritation caused by insects in 

 visiting the flowers. 



Neither of these explanations appears fully to meet the 

 difficulty presented by fruits adapted to birds. In berries 

 the softening of the fruit appears to have gone on simul- 

 taneously with the hardening of the seeds. In drupes the 

 outer layer of the pericarp has gi'own soft while the inner 

 layer has at the same time become indurated. It is by no 



means obvious how this dift'erentiation of tissues could 

 have originated. If we might hazard a conjecture, it 

 would be that succulent fruits are related to those having 

 an elastic pericarp which splits and forcibly ejects the 

 seeds within The latter we take to represent a more 

 primitive type. Hygroscopic ft'uits which in drying con- 

 tract and suddenly explode, scattering their seeds in all 

 directions, are not uncommon. Hygroscopic agency is 

 indeed very generally employed as a means of dispersion ; 

 we find it adopted in all classes of plants. The twisting 

 of the pods of the broom and the lotus, the spreading of 

 the plumes composing the thistledown, and many similar 

 phenomena, result from slight differences of adjoining 

 tissues which cause them to dry at dift'erent rates. A 

 piece of unseasoned timber warps in the same way 

 because the green sap-wood on the one side dries more 

 rapidly than the heart-wood on the other. The tur- 

 gescence of certain tissues producing tension and sudden 

 rupture accounts for the power of the mortar-fimgus to 

 project its spores. By this means some fungi literally 

 bespatter with their sticky spores the leaves and stems of 

 surroimding plants. The squirting cucumber when ripe 

 breaks away from its stalk, and, by the forcible contraction 

 of its walls as well as by the pressure of fluid witliin, the 

 seeds along with the watery contents are shot out as from 

 a syringe. 



Better known examples of elasticity are the fruits of 

 the hairy cress and the balsam. If these ripe fruits be 

 touched the valves or carpels suddenly curl up and the 

 seeds are thrown out all aromid. 



On the assumption that birds at first simply fed on 

 seeds, and that certain fruits and seeds already showed a 

 tendency to become differentiated into an external soft 

 and an internal hard layer, possibly as a means of scatter- 

 ing the ripe seeds by hygroscopic action, it is not difficult 

 to see how there would be initiated a course of variation 

 and natural selection which, by accentuating the difference 

 between the two layers, w-ould ultimately secure the com- 

 jjlete protection of the seeds, and at the same time provide 

 an inducement in the shape of sweet pulp sufficiently 

 attractive to insure the services of the feathered tribes. 



THE LONDON BASIN. 



By Edward A. Martin. 



TO those who live in the suburbs of London, geology 

 has a special interest, as, by its assistance, one is 

 enabled to solve questions as to the composition 

 and sanitary qualities of the various soils in the 

 metropolitan area. Enquiries are often being 

 made by persons about to choose a house, as to which 

 neighbourhood is a healthy one to live in, and where one 

 can find a good gravel soil. It is to be feared, however, 

 that with the comparatively small area of gravel in the 

 suburbs of London, only a few can choose such sites, and 

 many have to jjut up with a clay soil. In that case, a house 

 on a hill, or on rising ground, is preferable to one in a valley, 

 or on flat ground, whilst the chalk hills which almost com- 

 pletely surround London would be found to att'ord a far 

 better site than any of the London formations. But in 

 London, with the exception of a very small district in the 

 south-east, the chalk does not appear at the surface at all, 

 being regularly covered by one or more formations belonging 

 to the Eocene age, of which representatives are occasionally 

 found in the South of England, c.;/., at Newhaven, Seaford, 

 Furze Hill, Brighton, and in Hampshire. The chalk, as it 

 approaches London from the south, most aggravatingly 



