AGENCIES OF SEED DISPERSAL 



353 



The seedlings of all cereals and of many other plants are 

 hypogeam 



In many epigean species, as in the beech, squash, and 

 mustard, the cotyledons as well as the rest of the embryo 

 increase in size at germination. They turn green and manu- 

 facture considerable new food before they wither and 

 disappear. 



Fig. 156. — Development of a squash seedling ; a, mature seed ; m, micropyle; hi, 

 hilum ; b, c, and e, stages of the seedling ; h, the hypocotyl ; p, an outgrowth of 

 the hypocotyl called the peg ; ar, the arch of the hypocotyl ; note in this stage 

 that the peg is pressed firmly against the lower half of the testa furnishing thereby 

 a resistance which enables the arching growth of the hypocotyl to draw the rest 

 of the embryo out from the seed coat ; d, the empty testa (seed coat) ; pi, plumule ; 

 ca, cotyledons ; pr, the tap-root ; sc, branches. — After Andrews. 



Some seeds have special structures which are of mechani- 

 cal help in the withdrawal of the embryo from the testa. 

 This is well illustrated in the squash. (Study Figure 156.) 



68. Agencies of Seed Dispersal. — The principal agen- 

 cies of seed dispersal are wind, water, and animals. Among 

 animals, man and all the various means of transportation 

 operated by him are to be reckoned. 



A. Wind. — Of all agencies of seed dispersal, wind is 

 by far the most important for the largest number of plants. 



