66 



KNOWLEDGE 



[April 1, 1891. 



set up a struggle for existence in which only the most 

 capable would survive. Fruits of this kind therefore 

 split in a regular and distinctive fashion in order 

 that the seeds may escape. And not only do the 

 pericarps prevent a crowded planting by thus splitting, 

 bi;t they frequently still further carry out this im- 

 portant object by ejecting the seeds with considerable 

 force. The methods by which this is achieved are most 

 curious and interesting. The legume of the Pea splits 

 along its two margins, the two halves falling away from 

 eai-h other and throwing off the seeds in various direc- 

 tions. The seed-cases of the Pansy and of the Violet 

 explode, scattering the seeds forcibly. In the Gorse and 

 the Broom a sudden burst of the pods and a sjiring-like 

 twist of their two halves effectually disperse the contents. 

 On sunny -July days the cracking sounds produced by the 

 bursting pericarps may be distinctly heard. The mature 

 fruit of F.chdIUum elaterium separates from its stalk and 

 ejects its seeds with great rapidity through the orifice left 

 by the rupture. The sporangia of many ferns (Bracken) 

 have an elastic ring which is probably intended for the 

 energetic dispersal of the spores. In certain pines the 

 scales of the cone, when thoroughly dried by the hot days 

 of the summer following that of its production, open with 

 a jerk, forcibly ejecting the winged seeds. Frequently a 

 number will burst together, and then the sound may be 

 heard at a great distance. In the expulsion of the seeds 

 of the Balsam (Impatiens) the contact of some outside 

 object is of advantage. The seed-case consists of one cell 

 with five valves, and, if touched by accident when ripe, it 

 at once bursts open, the valves coiling themselves violently, 

 and, springing from the stalk, scatter the seeds in all 

 directions. In the Poppy and the Snapdragon a stOl 

 larger share of the work of releasing the seeds falls to an 

 outer agency, for here the pericarp consists of a capsule 

 which opens along the top by valves that leave small 

 pores through which the seeds fall out when the capsules 

 are shaken by the wind. In all of these various methods 

 of the expulsion of seeds it would seem that they are due 

 to mechanical causes, and depend in most instances (/»(- 

 jmtuns excepted) upon a certain condition of dryness in 

 themselves, and upon the state of the surrounding atmo- 

 sphere. 



Passing from the modes in which dry ft'uits, consisting 

 of a number of seeds enveloped in tough pericarps, effect 

 dissemination, we have to consider next the means which 

 obtain among dry fruits whose seeds are not collectively 

 enclosed in a strong seed-case. Here, too, we find the 

 forcible ejection of seeds. Those of the Oat are scattered 

 with such energy that on a fine, dry day the snapping 

 thus caused is distinctly audible. But the most curious 

 pronsion possessed by seeds of tbis class for self-dissemi- 

 nation is the hygroscopic awn. In the Wild Oat {Avena 

 fatua), for example, there is attached to the glumeUa (a 

 small leafy structure connected with the seed), a spiral 

 awn covered with numerous fine hairs, and this awn has 

 the power of expanding when moist, and of contracting 

 when dry. Thus the attached seed is constantly on the 

 move with the changes in the weather, the hairs clinging 

 to any object met with, until germination or destruction 

 puts an end to its motion. The seed of Barley, too, is 

 provided with a similar awn, which is furnished with 

 minute teeth that point towards its apex. The seed, 

 when lying on the ground, naturally expands with the 

 moisture of the night, and contracts with the dryness of 

 the day ; but, as the teeth prevent its mo^•ing towards the 

 point of the awn, all motion must be in the direction of 

 the base of the seed, which wiU thus travel many feet from 

 the parent stalk. As a ready proof of tbis, an ear of 



Barley will, if placed seed uppermost in the coat-sleeve in 

 the morning, be found to work up to the arm-pit during 

 the day. A still more remarkable provision exists in 

 Erodium, a genus belonging to the Geranium order, by 

 which the seed buries itself. The fruit splits into five 

 cone-shaped seeds, at the base of which is a long awn or 

 filament. As the seed lies on the ground the awn remains 

 straight so long as it keeps moist, but when it gets dry 

 one side of the awn contracts, forcibly causing the upper 

 end to form a curve which brings its point against the 

 ground, and the apex of the conical seed downwards. The 

 lower part of the awn now commences to contract into a 

 spiral, causing the cone to rotate and to enter the earth 

 where the hairs which it bears, and which point upwards, 

 hold it fast. The spiral portion also enters the ground, 

 forcing the seed dowTiwards. Moisture now, instead of 

 reversing the effect produced by dryness, only continues it, 

 for the spiral coils, in trying to straighten themselves, are 

 held fast by hairs, and the result is that the seed is driven 

 deeper into the groimd. It is a notable thing, as Mr. 

 Francis Darwin has pointed out, that these burying con- 

 trivances are all of a similar nature, though belonging to 

 plants of widely separated orders. 



Having considered plants which possess special facilities 

 in themselves for dissemination, we come to those which 

 depend to a gi-eat extent upon outside agencies for their 

 dispersion. Of these agencies, the all-pervading instru- 

 mentality of the wind may be taken as naturally next in 

 order to the power of self-distribution. The exceeding 

 smallness of many seeds, not to speak of spores, admits of 

 their ready transport by the wind. In addition to tbis, 

 certain fruits are evidently intended for dissemination by 

 this agency, for they are furnished with downy tufts or 

 with wings which support them on the breeze. When 

 ripe for dispersion, the light, flossy seeds of the Dandelion 

 and the Thistle may be seen floating in considerable 

 quantities on the softest wind. These special appendages, 

 though designed to serve the same purpose, and though 

 often similar in appearance, vary greatly in their origin. 

 There are three kinds of wing-shaped processes which, 

 while the fruit is developing, take their rise from different 

 parts of the flower. The wings on the seeds of certain 

 species of the Pine Order arise from an outer layer of the 

 tissue of the scale ; those on the seed of Bhinonia muvh-uta 

 from the coat of the ovule, and those on the samara; of the 

 Elm and the Maple are developments from the pericarp, 

 which, of course, in simple fruits is the matured ovary. 

 Again, the silky tuft of hairs, or pappus, of the CompositiB 

 and of kindred orders, is a peculiar development of the 

 calyx; whilst the coma of the seeds of Asclepias and of the 

 Willow is a hairy growth from the testa. Aided by these 

 special formatims on seeds and fruits, the importance of 

 the wind in the work of dissemination is difficult to over- 

 estimate. 



Frequently the wind acts in conjunction with another 

 outside agent of dissemination — water. The wind strips 

 the vegetation of a district of its fruits and carries them 

 into neighbouring streams, to be caught perhaps by the 

 bend of a bank where they form a cjlony. Plants growing 

 by the banks of rivers will thus be distributed along the 

 course of the stream. Curiously enough, it will sometimes 

 happen that an Alpine plant will in this manner be 

 brought into a lowland district where the climatal con- 

 ditions are not favourable to its growth. It may flower, 

 but cannot produce seed, and it is only by the continual 

 renewal of the seed by the current that the species is able 

 to maintain its occupation of the uncongenial localitj-. 

 Certam seeds will, however, be borne by the current right 

 out to sea, where, with others which have been carried 



