RETROSPECT. 877 



introduced into the soil unawares with the seeds of other plants, and so grow in 

 places where they would scarcely ever obtain a footing without the interference of 

 mankind. 



Looking back over this chapter we are struck by the following points: — In the 

 first place, the commonest contrivances and adaptations are those which confer some 

 other benefit in addition to that of dispersing fruits and seeds. The kind of struc- 

 ture most often encountered in this connection serves first as a means of protecting 

 the flowers against unbidden guests of the animal kingdom and against injurious 

 climatic conditions, subsequently as a means of scattering the fruits, and lastly, is 

 instrumental in attaching them to a substratum and in promoting germination. 

 Moreover, it appears from what has been said concerning pappuses and tufts of hair, 

 that it is no rare thing for contrivances to be adapted equally well to the dispersion 

 of fruits and seeds by the wind, by water, or by animals. It is also of great 

 moment to observe that most, and perhaps all, Phanei'ograms exhibit two methods 

 of disseminating their fruits and seeds, one of which is adapted to considerable dis- 

 tances, whilst the other is confined to the immediate vicinity of the mother-plant. 

 The former may and does, as a fact, take place on a vast scale, but it depends upon 

 the circumstances of the environment over which the plant itself has no control. It 

 may, therefore, in some circumstances, be completely suppressed; in other words, 

 dispei'sion to a great distance may take place but does not necessarily take place. 

 Just as the best-made machine stands still unless its wheels are set in motion by an 

 impulse from without, so the development of the most perfect flying apparatus is of 

 no avail if there is an entire absence of wind at the time when the winged fruit is 

 ripe; nor do the strongest hooks serve as means of dispersion if no animals come 

 upon the scene. On the other hand, dissemination within short distances of the 

 mother-plant always takes place if wider dispersion fails. Supposing the fruits of 

 a Maple-tree, when ripe, are not blown far away by a strong gust of wind, they are 

 ultimately detached spontaneously, and drop with a gyratory motion to the ground 

 close by the tree which produced them. Again, in the case of the Squirting 

 Cucumber, should the expulsion of the seeds from a fruit be caused by the touch of 

 an animal, and the seeds stick to the latter's coat, they may be carried to a distance 

 of many miles, but if no animal happens to pass the spot where the Cucumber is 

 growing at the time when the seeds are ripe the latter are spontaneously ejected, 

 and the dispersion so eflPected does not exceed a few paces in distance. In the event 

 of the fi-uits of Cydatnen (see p. 873), which are borne on twisted claw-shaped 

 stalks, not being carried away by animals, they remain lying on the ground in 

 immediate proximity to the mother-plant and the seeds germinate in that situation. 



These examples, to which might be added many others, show that the same 

 law governs the contrivances adapted to the dispersion of fruits and seeds as was 

 foimd to apply to the pollination of stigmas (see p. 390). Every species of plant 

 exhibits some mechanism designed to bring about a cross with another species, or, 

 at any rate, with another individual. If such mechanism is unsuccessful, other 



