CH. IV, 10] FOLIAGE-BEARING STEMS 179 



10. THE VARIOUS FORMS OF FOLIAGE-BEARING STEMS 



Stems which perform the same primary function, of. sup- 

 port to the foliage, can yet differ greatly in aspect and 

 structure. These differences are connected with different 

 habits. 



The simplest form of foliage-bearing stem is found where 

 a vertical, cylindrical, unbranched trunk bears one set of 

 leaves towards its summit, as do most Palms (Fig. 30). 

 Such stems, which may be termed COLUMNAR, characterize 

 crowded vegetation, as in the jungles of the tropics and 

 the meadows of temperate regions. They often attain 

 great heights in proportion to their diameters, in which 

 case they exhibit very perfectly the hollow-column principle 

 of construction (page 118). Thus the Bamboo, practically 

 columnar though it bears small branches, presents an actual 

 hollow column of such great strength as to give it high 

 value for building and other uses in the tropics. 



The second form of stem is found where the leaves are 

 spread along slender horizontal branches radiating from a 

 columnar-co'nical trunk. In such cases the younger and 

 shorter branches are above and the older and longer below, 

 thus producing the very perfect cone-shape displayed in our 

 Fir trees when free to grow without interference (Fig. 122). 

 This form of stem, called botanically EXCURRENT, is associ- 

 ated particularly with the evergreen habit, in which the leaves 

 persist and do some photosynthetic work in the winter. 

 Evergreen trees are especially characteristic of high latitudes 

 and altitudes ; and it has been argued that the conical 

 form, with the upper branches progressively covering the 

 lower except near the tips, which readily yield to a weight, 

 renders such trees largely immune against damage by the 

 snow and ice of the climates in which those trees most 

 abound. When such trees are well spaced apart, the cone 

 form is often strikingly perfect from ground to summit ; but 

 when crowded together in forests, their lower branches are 



