UNUSUAL GROWTH IN THICKNESS 



57 



Another method of growth in thickness is found in the arbor- 

 escent Liliaceae, represented by the genera Dracaena, Yucca, and 

 Aloe. Here, either close to the growing apex, or remote from it 

 in the region of the permanent 

 tissues, a secondary meristem 

 is formed by the tangential 

 division of the cells of the 

 pericycl'e, which adds new 

 cells both inward and out- 

 ward. A part of the cells 

 added toward the inside be- 

 come differentiated into new 

 vascular bundles (Fig. 29), 

 and a part into new ground 

 meristem; while those, much 

 less in number, formed to- 

 ward the outside constitute 

 a secondary cortex. In this 

 way growth in thickness goes 

 on from year to year, and has 

 been known to produce in 

 Dracaena a stem diameter of 

 fifteen feet. In this instance, 

 however, the tree was esti- 

 mated to be six thousand 

 years old. 



Unusual Growth in Thickness. — Variations from the usual 

 modes of secondary thickening take place in several families 

 of Dicotyledons (Apocynaceae, Sapindaceae, Bignoniaceae, Cheno- 

 podiaceae, Amarantaceae, Phytolaccaceae, Papilionaceae, Nyc- 

 taginaceas), and, under the Gymnosperms, the Cycadaceae 

 and species of Gnetum. A type of frequent occurrence is where 

 the cambium ring soon ceases its activity and a new ring of 

 secondary cambium is formed by tangential division of cells 

 of the pericycle, and after this has laid down a zone of vascular 

 bundles surrounding those first formed it becomes inactive and 

 a new cambium ring giving rise to a new zone of bundles is 



Fig. 28. — Photomicrograph of cross 

 section of cornstalk somewhat older than 

 in Fig. 27. Compare with Fig. 27, and 

 notice that the number of vascular biindles 

 is approximately the same in both, and the 

 number of cells in the fundamental tissue is 

 approximately the same. Growth in Fig. 

 28 has been accomplished by^ the enlarge- 

 ment of the cells already present in Fig. 27. 

 a, epidermis; b, cortex and pericycle; c, c, 

 fundamental or ground tissue corresponding 

 to pith and medullary rays with vascvdar 

 bundles interspersed through it. 



