GROWTH IN THICKNESS OF ROOTS, ETC. 



69 



predominates. In roots which grow in thickness, the formation of periderm usually 

 occurs at an early period; and this always takes its origin deep in the internal 

 tissue. Inside the endodermis before mentioned, which envelopes the entire axial 

 cord, lies a layer of parenchyma, the so-called pericambium ; and it is in this 

 (according to De Bary) that the periderm of the root arises. Thus the whole of the 

 cortical tissue of the root dies oft", and a new cortical layer — i, e. a phelloderm — 

 is produced by the activity of the phellogen, on the outer side of which is formed 

 at the same time a periderm, consisting of cork. 



Mention may here be made, finally, of the peculiar growth in thickness of many 

 napiform roots, e. g. Radish, Turnip, &c., as well as of the tuberous swellings of 

 some shoot-axes, such as the Potato, Turnip, &c. These parts of the plant, edible on 

 account of their soft, thin-walled, non-lignified masses of tissue, owe (since they too 

 are thin and filiform at first) their later thickness and massiveness to the subsequent 

 growth in thickness brought about 

 by means of a cambium - layer. 

 Essentially, and considered purely 

 formally, the processes are the same 

 here as in the ordinary cases 

 where the cambium produces true 

 wood ; only here, in place of the 

 development of tracheides and libri- 

 form fibres, the formation of paren- 

 chyma predominates, and the ligni- 

 fication of the cells produced on the 

 inner side of the cambium-ring is 

 entirely suppressed, or affects in a 

 slight degree only the not numerous 

 vessels which traverse the secondary 

 wood consisting entirely of non-lig- 

 nified parenchyma. Occasionally, as 

 is known. Turnips and Potato-tubers 

 become woody : they are then tra- 

 versed by tough inedible fibres — i. e. 

 by actually lignified strands. 



Passing over the very numerous cases of abnormal growth in thickness* 

 in dicotyledonous woody plants (since these, in spite of considerable deviations 

 from the type, nevertheless differ in no way essentially from it), I may still say 

 a few words as to the growth in thickness of a small group of monocotyledonous 

 plants — Monocotyledons not elsewhere exhibiting this process. Here are concerned 

 a sub-division of the Liliaceae, to which the well-known genera Draccena, Yucca, and 

 a few others belong. These plants, palm-like in advanced age, possess when young a 

 thin stem, scarcely as thick as a finger, which may subsequently attain a considerable 

 thickness. This occurs of course by growth in thickness ; and this extends also into 



FIG. 174.— Transverse section of a thin root on the rhizome of the 

 Nettle (Urtica dioica). irthe series of primary vessels arranged right 

 and left. The secondary wood forms two groups— lying above and below 

 in the fijfure. c the cambium. The primary cortex lias been thrown off; 

 the periderm is developing at the circumference (after De Bary). 



1 The essential facts on abnormal growth in thickness are found in De Bary's 'Anat. da 

 Vcgetations-orgaiie^ Cap. XVI. 



