INTRODUCTION 5 



first structure to emerge from the seed. It grows 

 downwards. Then a curved-over stem structure 

 emerges below the cotyledons and grows upwards. 

 This structure is the hypocotyl. As the hypocotyl 

 grows the cotyledons gradually emerge from the seed 

 and appear above the soil. The hypocotyl then 

 straightens and the two cotyledons which have 

 become green unfold and assume a horizontal posi- 

 tion. The radicle has meanwhile grown longer, and 

 the root-hairs are very clearly seen. It is some time 

 before the true plumule develops in this case, a con- 

 siderable growth of root with branching having taken 

 place before the first true leaf unfolds. 



In this type of growth the cotyledons are said to be 

 epigeal, as they appear above the soil. 



In both the pea and mustard the first stem structure 

 to appear is curved and in both the same kind of root 

 system is formed. 



In the grain of wheat (see Fig. 3) the radicle 

 emerges first ; its appearance is very shortly followed 

 by that of one or two other root-structures which 

 appear at the side and might be mistaken for branches. 

 If, however, growth is allowed to continue it will be 

 seen that the number of these root-structures in- 

 creases, and if the germinated grain is carefully dis- 

 sected it will be seen that these structures arise above 

 the radicle on the main axis of the embryo. These 

 roots are, therefore, adventitious. In a very short 

 time it is not possible to distinguish the radicle among 

 the numerous roots, and thus a root-system results, 



