FLATTENED SHOOT 181 



similar movement at night appears to have for its object 

 the protection of the green tissue against chill. 



OPUNTIA DILLENII, Haw. 



The Prickly-pear. 



At first sight, this plant seems to be made on an 

 utterly different plan from the ordinary. It has normal 

 roots but the shoot instead of consisting of cylindrical 

 stems and branches, with flat green leaves, seems to 

 be made up of a number of very thick flat oval seg- 

 ments fixed end to end. These segments are studded 

 with little areas, from which project numerous short 

 barbed hairs (the fact that they remain so firmly stuck 

 in the human skin shows that they are barbed) and 

 one or two long yellow thorns. Occasionally on the 

 uppermost segments, especially during rains, there are 

 to be seen short thick green leaves, and, above each, 

 one of these little thorny areas, which therefore are 

 axillary to the leaves. These leaves soon fall off, 

 so that most of the plant, and indeed the whole of 

 the plant for much of the year, has no leaves at all. 

 And this is why the stem is green to make up for 

 the absence of the leaves, and it is flattened for the 

 same reason that leaves are flat, to expose the greater 

 surface to the air and light. 



Each segment of the shoot represents a portion of 

 the axis, for the shoot does not grow straight in 

 monopodial growth, but after a few weeks, the direct 

 growth ceases, and is continued by a lateral branch. 

 This is the reason for the segmentation of the shoot, 

 and for the irregular way in which the segments are 

 attached to each other. 



