GYRATroy OF PLANTS. 155 



the actual apex exerts relatively to its diameter the 

 same transverse strain as the parts a little higher up ; 

 but there seems no reason to doubt that this would 

 be the case. The growing part, therefore, does not 

 act like a nail when hammered into a board, but more 

 like a wedge of wood, which whilst slowly driven into 

 a crevice continually expands at the same time by 

 the absorption of water, and a wedge thus acting will 

 split even a mass of rock." ^ 



From the seed upwards rises the short rudimcntar\' 

 stem which supports the cotyledons, or seed-leaves. 

 This short stem (if present) raises the cotyledons 

 above the surface of the soil through which it breaks 

 in the form of an arch. When the cotyledons, or 

 seed-leaves, appear, they are at first vertical, with their 

 faces applied to each other ; but they soon separate 

 and exhibit the phenomena of rxjtation. In all the 

 .seedlings of dicotyledonous plants examined, the 

 seed-leaves were in constant movement, chiefly in a 

 vertical direction, and most often once up and down 

 in twenty-four hours. In one plant they moved 

 upwards and downwards thirteen times in the course 

 of sixteen hours, but this is unusual. In different 

 species and in different individuals of the same 

 species, there will be variation and gradation in the 

 oscillations. In one species of wood - sorrel, whilst 



* Darwin, " Movements of Plants," p. 77. 



