SEEDS AND SEEDLINGS 



143 



In dicotyledonous seeds of the type of the pea, the horse- 

 chestnut, and the buckeye, the cotyledons remain inclosed in 

 the seed coat and underground 

 (Fig. 132), where they become 

 emptied of their contents. They 

 are so loaded with reserve ma- 

 terial that they could not serve 

 any useful purpose if they were 

 to emerge into the air and light. 



In the bean, they 

 are raised into the air, 

 turn green, develop 

 stomata, and probably 

 for a short time do 

 some photosynthetic 

 work, but soon wither 

 and fall off. 



In the squash, pump- 

 kin, and most dicoty- 

 ledonous plants of the 

 farm and garden, the 

 cotyledons become for 

 a considerable time 

 active green leaves, 

 but they are shorter- 

 lived than the sub- 

 sequent leaves of the 

 plant, are opposite or 

 nearly so (while the 

 later leaves may be 

 opposite or alternate), 

 are usually smaller 

 than other leaves, and 

 their shape always 

 differs from that of the permanent leaves of the plant. Even 

 in those cases where the cotyledon for a time becomes wholly 



FIG. 133. Two stages in the growth of the bean 

 seedling 



In the younger stage the arch of the hypocotyl is 

 but little above the surface ; in the older stage the 

 cotyledons have separated, the first internode has 

 elongated considerably, and the first pair of foli- 

 age leaves has expanded. Cot, cotyledon; h, hy- 

 pocotyl ; ha, hypocotyl arch ; i, internode ; I, leaf ; 

 n, taproot which proceeded from the tip of the 

 hypocotyl; r 2 , branches of n. Natural size 



