GROWTH IN THICKNESS OF ROOTS, ETC. 



l6g 



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 off, 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 Dracmna, 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). ^the series of primary vessels arranged right 

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

 in the figure, c the cambium. The primary cortex "has been thrown off; 

 the periderm is developing at the circ'uibference (after De Bary). 



> The essential facts on abnormal growth in thickness are found in De Bary's 'Anaf. der 

 Vegetations-organe,' Cap. XVI. 



