CH. II] BEAN. 19 



early in their development we can draw a transverse line 

 across the embryo which shall divide it into two distinct 

 morphological regions, a point which will be more clearly 

 realised when the embryology of plants is studied. 



In the growth of the seedling bean, the first thing that 

 happens is the elongation of the radicle: it is not until 

 the radicle has grown considerably that any striking 

 development of the plumule takes place. This order of 

 growth has a clear biological importance; the young 

 plant must get a hold on the soil before it can raise a 

 structure such as a stem above the ground. 



An interesting fact about the plumule is its hook-like 

 form. When a bean is planted beneath the surface of the 

 ground, the part of the plumule which emerges is the 

 curved outline of the hook : it pushes its way through, and 

 makes a path for the delicate tip of the plumule which 

 follows it. If the plumule were straight, the tip would 

 have to make its own way through the soil at the risk of 

 being injured. 



The most striking fact about the cotyledons of the 

 bean is that although they are undoubted leaves, they 

 never assume the appearance or functions of ordinary 

 leaves; they do not become green, and they are never 

 expanded in the air and light, nor do they increase in size l . 

 Without growing themselves, they give up their accumu- 

 lated reserve material to the radicle and the plumule. 



It is not necessary to consider the nature of all the 



1 The only growth is that of the stalks of the cotyledons, by which 

 the plumule is freed from its position between the cotyledonary lobes 

 and enabled to grow freely upwards : see fig. 4, C, c. 



22 



