8 



PLANT GROWTH 



row layer in the outer row of cells, which is impervious to 

 water. A few other seeds have seed coats so tough that the 

 seedling cannot break it to escape. If seeds do not swell with 

 a few hours of soaking it is best to break the seed coat, since 

 it so frequently is a cause of delayed germination. Some 

 seeds of wild plants have their germination delayed several 

 years by their naturally tough seed coats, and thereby some 

 of them may germinate at a more fortunate time for their 

 survival than if all germinated at the same time. 



The hilum is the scar on the seed coat which was caused 

 by the attachment of the seed to the ovary, and through 

 which the food passed to the seed. This is very obvious in 

 bean seeds (Fig. 2). The micropyle (Fig. 2) is a small pin- 



MICROPYLE 

 HILUM 



HYPOCOTYL 

 RADICLE 



PLUMULE 



Fig. 2. Seed of bean. A, side view. B, as seen from inner (attached) edge. C, 

 from the outer edge. D, embryo; seed coat and 1 cotyledon removed. 



point pore below the hilum through which the pollen tube 

 entered and toward which the radicle points inside the seed 

 coat. 



Every seed must grow from a fertilized ovule in a flower, 

 as described in more detail in Chapter 17. In the common 

 garden pea each flower has an ovary which will develop into 

 the mature pod. Each pea or seed in the flowering stage was 

 only a small ovule consisting of a mass of cells with a single 

 female sex cell. A pollen grain must fall on the stigma and 

 grow the pollen tube bearing two male sex cells, one of which 

 must unite with the female sex cell of each ovule. This is 

 called fertilization and results in a powerful stimulation for 



