320 



BOTANY 



agreeably tasting and often attractively coloured fruit, and excreting tin- un- 

 digested seeds, or by their involuntary transportation of seeds and fruits which 

 have become in some way attached to them. This is effected in many cases by 

 hooks and bristles (Lnppa, Galium aparinc, B-ulciix, 7:>A ///"*/;<//// //w. Xiniiliinm, 

 and the fruits <>f M-il-icago minima, so common in sheep's wool and erroneously 

 termed wool-lice). The seeds may also become attached to animals by means of 

 some sticky substance ; in this way the seeds of the Mistletoe, which stick to the 

 1 leaks of birds eating the berries, finally adhere to the 

 brandies of trees upon which the birds wipe their 

 bills. The widespread distribution of fresh-water 

 plants can only be accounted for through the agency 

 <>!' aquatic birds. 



The natural distribution of plants has been greatly 

 modified by the interference of man, especially in these 

 days of universal commercial intercourse by rail and 

 sea. By their instrumentality not only have the 

 useful plants been widely distributed over the earth, 

 but the weeds have followed in the same way ; and 

 many a seed thus accidentally carried to other lands 

 has finally found there a new place of growth. 



The forcible discharge of spores and seeds is effected 

 by the sudden liberation of hygroscopic or tissue 

 tensions. It has already been mentioned that the 

 eapillitia of the Myxomycetes and the elaters of 

 the Liverworts serve for the dispersal of the spores. 

 In the case of the Box (Bumis), the smooth seeds 

 are forcibly discharged by the contraction of the 

 pericarp, like a bean pressed between the fingers. 

 The dry fruit of Hura crepitans bursts apart with a 

 report like that of a pistol, and is scattered in pieces 

 far and wide. The turgescence and elasticity of the 

 cell-walls give rise to the tension which results in the 

 tto upper ^ecimea the corky forcible abjection of the sporangia of Pilobolus (Fig. 

 floating tissue is seen from the . ,. 



outside, the smooth, super- 224 )> and m the ejection ot the ascospores ot many 

 ticial layer being already worn Ascomycetes. The bursting and rolling up of the 

 away. The lower figure is segments of the seed-vessels of Liipaticiin, by means 

 of a cross section through of which the <ji spersa i O f t h e 8e eds is effected, are due 

 the floating tissue ; the seol , . . - . . _,. ,. . 



has been removed (\fter to t ' ie su dden release of tissue-tensions. The fruits 

 S( JIIMPER.) of Momordica elaterium and Ecballium have the 



pericarp distended by the pulpy mass within ; when 



the mature fruit becomes detached from the stalk which fitted like a champagne 

 cork into its place the fluid contents with the seeds are forcibly squirted from 

 the opening. 



Germination. The first condition for successful germination is 

 that the seed should retain its vitality during its dispersal. The 

 small amount of water contained in the seed and the arrest of the 

 main processes of life render the resting seed resistant both to ex- 

 tremes of temperature and to drying. Firm investments serve to 

 protect it from mechanical injury. The second condition of germina- 

 tion is that the seed should reach suitable soil, and this is effected by 



Flo. 249. Fruit of Termiiialia 

 cctiappa from the drift. In 



