THE STEM 165 



becomes reunited with that situated in the center of the stele, 

 designated in turn as the medulla or pith. 



Fig. 119 depicts a siphonostelic central cylinder. The traces of 

 leaves determine the presence of lacunae, or interruptions in the 

 continuity of the cylinder, for some distance above their points 

 of departure. These are the foliar gaps or, in case they are related 

 to lateral branches, the branch gaps. 



The protostelic and siphonostelic conditions of stem are both 

 subject to further modification which may now be discussed. In 

 the protostelic stem, for example, the phloem may more or less 

 completely disappear on one side of the stele, a situation found in 

 the stem and sometimes even in the root of certain ferns. This 

 condition is known as collateral and is extremely rare in proto- 

 stelic organs. This state of reduction is, however, very common 

 in the siphonostele, particularly in the higher forms where the 

 tubular type of central cylinder has become universal. A proper 

 understanding of the phenomena of reduction presented by the 

 siphonostele is advantageously gained by the consideration of allied 

 forms presenting the stelar conditions both in the normal and in the 

 reduced state. Illustrations of this nature can best be drawn from 

 the lower vascular plants, the ferns and their allies. Fig. 120 

 represents on the left a siphonostelic stem in which internal and 

 external phloem are both well developed, and likewise the endo- 

 dermal layers, which respectively limit these from pith and cortex. 

 In the illustration at b appears a stem in which the internal phloem 

 has disappeared on most of the interior of the stele and is found 

 merely on the margins of the foliar gap. In the case of the chan- 

 neled leaf trace, subtending the foliar gap, in contrast to the 

 situation presented by the fibrovascular system of the stem, the 

 internal phloem is as well marked as that occurring on the outer or 

 convex surface. This is in accordance with a general principle 

 to be more definitely elaborated at a later stage. The basal region 

 of the foliar fibrovascular strand in vascular plants is found in 

 general to retain the more primitive type of organization. In the 

 particular instance under consideration the retention of internal 

 phloem in the leaf trace is the significant feature in a consider- 

 able number of ferns and fern allies. It may be stated that a, 



