14 PLANT GROWTH 



direction of growth has recently been found to be due to the 

 growth hormones, which are discussed in Chapter 20. 



Soil frequently forms a crust on the surface through 

 which the shoot must force its way. Several methods of 

 protecting the tender growing cells may be observed, as 

 shown in figures 4 and 5. The common bean and hollyhock 

 push the cotyledons through the soil by the elongation of the 

 hypocotyl, with a sharp bend at the top, before the tender 

 plumule emerges. The garden pea pushes the plumule up- 

 ward in a shepherd's crook position protecting the growing 

 tip, while the cotyledons remain where the seed was planted. 

 In the grasses, as is illustrated by oats, the cotyledon remains 

 in the endosperm while a tough sheath, called the coleoptile, 

 fits over the end of the growing shoot until it emerges, after 

 which the sheath splits to free the plumule. Most albumi- 

 nous seeds other than the grasses germinate similarly to the 

 common bean and bring the endosperm into the air with the 

 cotyledons before the plumule appears. 



The primary root grows secondary roots and in most 

 cases is the beginning of the root system of the mature plant. 

 The tap root is a continuation of the radicle in most cases. 

 In the grasses a permanent root system develops later at a 

 uniform distance from the soil surface regardless of the depth 

 of planting. When the cotyledons come above the ground 

 the hypocotyl, meaning the part which is below the coty- 

 ledon, has a structure unlike the older shoot. The cotyledons 

 of some plants, such as the tomato, shown in the photograph 

 (Plate I), become green, grow and make food, but many 

 others lose their food to the growing plant and gradually 

 shrivel and later fall off. The seedling stage is considered 

 past when the plant no longer depends on food supplied from 

 that stored in the seed. 



