172 THE MICEOSCOPE AND ITS LESSONS. 



typical of exogens, whose growth is from without, and 

 the last of endogens, whose growth is from within 

 represent the greater part of the family of trees, in 

 the varieties of which there are so many thousands. 

 The screw pine family, from which this latter specimen 

 was taken, affords another opportunity of observing 

 what is called "a law of nature," but which is very 

 strikingly illustrative of Divine wisdom. The foliage 

 is peculiar, its great sword-shaped leaves resembling 

 those of the pine-apple, set with sharp spines along each 

 edge, running in regular spiral turns like a corkscrew, 

 whence the common name of the tribe, Pandanus spiralis. 



The screw pine is frequently found growing in the 

 loose shallow sands of the coral islands which dot the 

 Pacific Ocean ; and, as they are inclined to be " top- 

 heavy," they have "acquired" a very demonstrative 

 method of attaching themselves to the loose soil in which 

 they grow, so as to enable them to resist the force of the 

 furious winds that beat upon them. When the leaves 

 are abundant, and the tree would fall by reason of their 

 excessive weight, the aerial roots which would ordinarily 

 have grown straight downward to the earth, issuing from 

 various parts of the stem, when the tree is full-grown and 

 is overloaded, proceed in a slanting direction, and, firmly 

 fixing themselves in the ground, surround the trunk, 

 shoring it up just as a carpenter would a falling tenement. 



Some years ago a large tree belonging to the family 

 of screw pines, in the palm-house of the Edinburgh 

 Botanical Garden, had one of its branches injured, and 

 at the point of injury a root appeared long before its 

 time, and thus supported a branch which would otherwise 

 have been cut off.* 



These wood sections will afford us inexpressible 

 * Professor Balfour. 



