CHAPTER XIII 

 THE STEM 



In the older types, represented by the vascular cryptogams, the 

 general organization of the stem presents more variety in ground 

 plan than that found in the case of the seed plants. In these earlier 

 forms the fibrovascular tissues are present in two main conditions. 

 In the primitive type of structure the xylem is a solid mass which 

 does not include any medullary substance or pith. This variety 

 is represented in Fig. 118 and is known as protostelic. The 

 protostelic condition is found in many older groups and is of almost 

 universal occurrence in the most conservative of all the organs of 

 the plant, the root. Where the mass of primary xylem is smooth 

 in contour and does not show on its surface any projecting salients 

 of protoxylem, the phloem forms a continuous layer surrounding 

 it, and the whole fibrovascular complex so organized is known as 

 concentric. Usually in concentric protosteles the protoxylem 

 elements are situated in relation to the general organization of the 

 xylem in the manner described in an earlier chapter as mesarch. 

 Here it will be recalled that the first-formed elements of the 

 wood are ultimately completely surrounded by those of later 

 origin. When, by reason of the projecting masses of protoxylem 

 not imbedded in the main mass of the primary wood as in 

 the mesarch type, there are more or less deep furrows on the face 

 of the xylem, the phloem ceases to constitute a continuous layer, 

 but is present in isolated strands alternating with the projecting 

 angles of the protoxylem, and the stelar structure is described as 

 radial in its organization. In this condition the development of 

 the wood is usually entirely centripetal, or toward the center of 

 the organ. This mode of organization is anatomically described as 

 exarch, because the protoxylem elements are external in position 

 to all the later-formed portion of the wood. 



Another primitive type of structure presented by the fibro- 

 vascular organization of the stem is that known as siphonostelic. 



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