SECT. II PHANEROGAMIA 523 



distribution of water-plants from one pond to anotlier by aquatic birds, and the 

 distribution of the Hazel-nut, etc., by means of squirrels do not require detailed 

 description. 



It is a matter of general knowledge that man by his commerce and industry 

 has exerted great influence on the distribution of food-plants and other plants of 

 economic value. With these the seeds of many weeds have been unintentionally 

 distributed over the inhabited earth, a fact that could be illustrated by numerous 

 examples. 



The active movements of plants by which their seeds or spores are forcibly 

 ejected, whether these movements are due to cohesion, turgescence, or are hygro- 

 scopic, have been already discussed (cf. p. 296). 



Germination 



Seeds which have escaped the various risks of distribution require 

 to be soon covered with soil. Small seeds readily find shelter in 

 cracks or depressions of the soil and become fixed there owing to 

 special properties of their surface. Larger seeds are sufficiently 

 covered by fallen leaves. The fruitlets of Erodium and other Gerani- 

 aceae, of Avena sterilis, species of Stipa and other Gramineae penetrate 

 the soil by the aid of their hygroscopic curvatures ; the presence on 

 their surface of backwardly directed hairs prevents their losing the posi- 

 tion reached. The burial of the fruits of Arachis hypogaea, Trifolimn suh- 

 terraneum and Okenia hypogaea is brought about by the growth of their 

 positively geotropic stalks, while negative heliotroj^ism determines the 

 insertion of the fruits of Linaria cymbalaria into the crevices of the 

 walls on which the plant lives. 



When the seeds find sufficient moisture they swell considerably. 

 With this they lose some of their resistance to such dangers as 

 extremes of temperature and dessication ; their former resistance was 

 due to the small proportion of water they contained. The next step 

 is the rupture of the seed-coat, which is effected by the emerging root, 

 less frequently, as in some Monocotyledons, by the cotyledon. The 

 root at once bends downwards geotropically and by means of its root- 

 hairs, which are especially long and numerous at the junction of the 

 root and hypocotyl, fastens the seedling in the soil. Meanwhile the 

 hypocotyl grows and gradually emerges from the seed-coat, while the 

 cotyledons as a rule remain for a time enclosed in the latter and 

 absorb the remainder of the reserve material (Fig. 494 a). This 

 process leads to the hypocotyl becoming more and more strongly 

 curved and the tension resulting from its further growth withdraws 

 the cotyledons from the seed-coat. The seedling then becomes erect, 

 the leaves are expanded and can assimilate, and thus its independent 

 life commences. • 



This most frequent type of germination is characterised by the 

 cotyledons being expanded above ground and is termed epigeal. It 

 is nearly always found in the case of small seeds. Further details 



