Trees of New York Si ate 25 



cell walls arc rciKlered harder, stroiifier and more durable than 

 before. All Avoody tissues become more or less lignified the first 

 year, soon after they attain their ultimate grrowth, and the process 

 should not be confused with the changes which occur in passing 

 from sapwood to heartwood, Lignification is in no sense confined 

 to the so-called "woody plants", or, in fact, to vascular tissue. 

 Woody stems possess in proportion more tissue that is lignified 

 than herbaceous plants, and hence seem woody to us. 



5. Typical woody plants possess secondary thickening, that is, 

 have a means of thickening their stems by subsequent growth in 

 diameter which is not traceable to terminal growing points. This 

 is achieved through the activities of a growing layer or cambium 

 A\hich is situated just oiTtside the last formed layer of wood and 

 beneath the bark, and produces new wood and new bark yearly 

 which are interpolated between the older wood and bark. This 

 results in the formation of the annual rings which are character- 

 istic of cross-sections of the trees of temperate regions. Tropical 

 trees are often devoid of annual rings because cambial activity 

 extends over practically the whole year and the resulting wood 

 is quite homogeneous. 



But there are arborescent ferns and monocotyledons (palms) 

 which are devoid of secondary thickening of the normal type, in 

 that the woody tissue is not gathered together in a cylinder sur- 

 rounded by a cambium but is scattered through the stem in the 

 form of isolated vascular bundles. In such arborescent forms 

 subsequent seasonal increase in the thickness of the stem, where 

 it occurs, is due to the continued enlargement, over a period of 

 years, of tissues which had their inception in the apical growing 

 point. This explains the fact that many palms support but a 

 given number of leaves in their crown and new leaves develop 

 only in proportion as some of the older leaves cease to function. 

 In other cases, monocotyledonous stems increase in girth through 

 anomalous secondary thickening, that is, not in the typical way. 

 Finally there are many woody monocotyledons, especially lianas, 

 which exhibit little or no secondary thickening, as in the case of 

 Smilax hispida Mohl., the Hispid Greenbrier. 



KINDS^OF^WOODY PLANTS 



Woody plants are of three sorts, (1) trees, (2) shrubs, and (3) 

 lianas, between which no hard and fast lines can be drawn. A 

 given species may be shrubby near the limits of its range and 



