THE LIFE AND GROWTH OF SEEDLINGS 



ii 



an arch as that of the pea or bean, for as the plumule is not 

 situated between two cotyledons, there is not the same necessity 

 for a vigorous pull. On the other hand, the onion does make an 

 arch. Darwin notes (Movements of Plants, p. 87) that in germina- 

 ting monocotyledonous seeds, the plumules are straight whilst 

 breaking through the ground, and that those of the Gramineae are 

 likewise straight ; their emergence from the soil is facilitated, 

 he thinks, by the sharp crest in which they terminate. In the 

 onion, the crown of the arch is developed into a white, conical 

 protuberance which he considers helps the seedling to break 

 through the ground. 



The growth of the onion seedling is shown in Fig. 5. By 

 gently scraping away the soil the grain is seen, and the 

 arch form of the plumule. The 

 level of the soil is indicated by 

 the dotted line. As the seedling 

 grows it frees itself from the 

 grain, but sometimes this is carried 

 up above the soil, as in the 

 illustration. As the plumule 

 grows, it gives off leaves which 

 ensheath the stem. The root- 

 system does not show a tap root, 

 for in monocotyledons the prim- 

 ary root does not grow to any FlG - 6. Seedlings of cucumber; r, 

 great length; the Other roots radicle; A peg of radicle; ,, cotyle- 



dons ; /, seed-coat. 



soon equal it. A root-system of 



this kind is described as fibrous. The primary root can usually 



be detected by the sheath at its base. 



The pumpkin and cucumber have a peculiar way of 

 pulling the cotyledons out of the seed-coat. The general 

 structure of the seed is similar to that of the bean ; the 

 radicle pushes its way out first and grows down into the soil. 

 On its lower side it develops a peg, which holds down the 

 seed -coat until the cotyledons have burst through the testa. 

 Cucumber seeds must be grown under glass, and are very 

 slow in developing, although, of course, much depends on the 

 temperature. 



