DIV. n 



SPERMATOPHYTA 



seeds by means of ants must be mentioned ; these animals are attracted to remove 

 and accumulate the seeds by the abundance of oil in the elaiosome-containing 

 tissue of appendages such as the caruncula. 



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. In this way the seeds of many weeds have been unintentionally 

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

 examples. 



Germination ( 1S ) 



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 (cf. p. 

 334, Fig. 275) ; the presence on their sur- 

 face of backwardly-directed hairs prevents 

 their losing the position reached. The 

 burial of the fruits of Arachis hypoguea, Tri- 

 foliiim suUcrraneum, and OJcenia hypogaea 



) 



FIG. 577. Thuja occidental is. A, Median longi- 

 tudinal section of the ripe seed. B-E, Stages 

 in germination ; h, hypocotyl ; c, cotyledons ; 

 r, radicle ; v, growing point of stein. (A x 5 ; 

 B, C x 2 ; D, E nat. size. After SCHENCK.) 



FIG. 578. Pinus pinea. Germination. 

 (After SACHS.) I, Longitudinal sec- 

 tion of the seed ; y, micropylar end. 

 II, Early stage of germination ; s, 

 seed-coat ; e, endosperm ; to, primary 

 root; x, broken -through embryo-sac ; 

 r, red layer of the seed-coat. Ill, 

 The cotyledons (c) have escaped from 

 the exhausted seed ; he, hypocotyl ; 

 /, lateral roots. 



is brought about by the growth of their positively geotropic stalks, while negative 

 heliotropism determines the insertion of the fruits of Lin aria cymbalaria into 

 the crevices of the walls on which the plant lives (cf. p. 351). 



When the seeds find sufficient moisture they swell considerably. With this 



