EMERGENCE OF SEEDLING 437 



long encased. We may note a remarkable feature of the 

 marrow and other Cucurbitaceae, where a peg of tissue is 

 formed on the lower side of the hypocotyl (gravitational 

 induction), which levers open the seed coat so that the 

 cotyledons are freed. 



Seeds may germinate on the surface of the ground, but 

 we look upon subterranean germination as the normal. 

 How the radicle pushes its way through the soil we have 

 already seen ; the conditions for the shoot are different. 

 It usually bears at its tip a tender bud, and it may be encum- 

 bered with the enlarged cotyledons ; in the emergence of 

 the shoot damage is easier, and frictional resistance is 

 greater than with the radicle. The lessening of resistance 

 and the protection of the delicate plumule are achieved in 

 a variety of ways. 



Perhaps the simplest case is seen in cereals such as the 

 oat. The first leaf, the coleoptile, is a white, closed sheath 

 completely enclosing the younger organs and never forming 

 chlorophyll. Its tip is rather sharp, and, in the dark, it 

 may attain a length of 3 to 5 cm. before it ceases to grow. 

 In fact, it has much more the appearance of a radicle than 

 of a leaf, and it penetrates the soil, growing upwards, 

 nutating slightly, and very sensitive to contact stimulus, 

 much Hke a root tip. Normally it reaches the surface before 

 the next leaf, completely protected in its passage through 

 the soil, bursts through its tip. In a deeply buried seed the 

 first foliage leaf may have to make its own way through the 

 soil. While this leaf is in the dark it elongates rapidly and 

 remains narrow and rolled into a tube, thus oifering a mini- 

 mum resistance. The importance of this type of etiolation 

 for easy penetration may be shown experimentally. If seed 

 is sown deeply in soil against the glass side of a suitable 

 box, so that the young plant, though still buried, is early 

 illuminated, the first leaf, as soon as it appears, forms 

 chlorophyll, flattens out, and increases in breadth. It 

 cannot make its way upwards ; the friction of the soil keeps 

 it down and it becomes twisted and folded. 



An interesting case is that of the onion. After the 



