CHAPTER XIV. 

 SECONDARY GROWTH IN THICKNESS OF STEMS AND ROOTS. 



A. NORMAL SECONDARY THICKENING. 



I. GENERAL CONSIDERATIONS. 



It is characteristic of all young stems and roots, that the region which 

 is in process of conversion into permanent tissue is much thicker than 

 the portion nearer the apical growing point, in which the primary 

 meristematic layers originate from the primordial meristem. This 

 increase of diameter, which accounts for the conical form of the 

 apical region, is the result of the so-called primary growth in thickness, 

 and does not depend upon any special growth-process ; every cell 

 in any cross-section takes some part therein, though all do not partici- 

 pate to an equal extent. 



This primary growth in thickness, or " enlargement " {Erstarkung), 

 as it may also be termed, determines the average diameter characteristic 

 of the several organs of any given species, in their primary condition. 

 "When this thickness has been attained, the organ begins to grow 

 in length. In the majority of Monocotyledons and Vascular Crypto- 

 gams, no appreciable growth in thickness takes place, once elonga- 

 tion has ceased and the primary permanent tissues, have become 

 completely differentiated. In most Gymnosperms and Dicotyledons, 

 on the other hand, in the Palms, Paisdanaceae and arborescent Liliaceae, 

 and in a few Vascular Cryptogams, the stems, and, as a rule also, 

 the roots, are capable of further growth in thickness, even after 

 longitudinal extension has come to an end and tissue-differentiation 

 is completed. 



According to Eichler and Barsickow, 34 ' the <: secondary thickening " 

 of certain Palm-trunks, which continue to increase in girth for a consider- 

 able time, really consists of a protracted " enlargement " of the stem. 

 In such Palms, therefore, the diameter of the trunk increases steadily 



