81 



exfoliated. In this way a thick corky bark may be formed. 

 As a rule, however, its activity ceases after a short time and 

 another, or secondary, phellogen arises deeper in the stem 

 beneath the first and nearer the actively growing cambium 

 layer. In this case again all the living cells outside the second- 

 ary phellogen die and are added to the dead outer bark just 

 as happens when the primary phellogen develops deep in the 

 fundamental tissue. In this way a number of successive peri- 

 derms may be formed, succeeding each other at short intervals. 

 It will thus be seen that the outer dead bark may consist of 

 cork cells only or. in addition, of dead elements which originally 

 formed a part of the primary fundamental tissue, or of successive 

 periderms, or even of the phloem developed from the cambium. 

 Taking also the word cortex, or bark, in the usual sense as- 

 including all the tissues outside the cambium ring, it will be 

 seen that this consists of an outer portion of dead elements and 

 an inner part which is largely composed of living cells containing 

 protoplasm. According as the periderm forms a regular circle 

 around the stem, or only arcs of a circle, so do the outer layers 

 of dead bark exfoliate as rings or scales.* 



Dead bark which does not exfoliate rapidly becomes deeply 

 cracked and fissured. 



In the roots of Dicotyledons and Gymnosperms which 

 exhibit secondary growth the primary phellogen usually origi- 

 nates close to the primary phloem, all the tissue external to it 

 then peeling off, while subsequent phellogens arise as in the stem. 

 In the stems and roots of those Monocotyledons which possess 

 a cambium ring and which therefore increase in thickness^ 

 periderm is also formed outside the cambium ring in the way 

 just described only in this case the cambium, instead oi 

 producing bast continuously on the outside, gives rise to new 

 fundamental tissue. 



77. At certain points in both 

 roots and stems the phellogen, instead of producing cork cells 



* In this book the word '' bark " is used in its ordinary sense and denotes all 

 tissues situated outside the cambium. In some botanical books it is applied in a res- 

 tricted sense to the dead tissues situated outside the phellogen. For the latter, however, 

 the word rhytidomc seems preferable. 



The word rJiytidome appears to have been first employed in Indian Botanical 

 literature by W. R. Fisher in his Morphological Botany (Roorkee 1888), but it has 

 not as yet been generally adopted by botanists. In that book Mr, Fisher has 

 spelt the word in two ways rfiytiderm (page 76) and rJiytidome (page 12C). In his 



Tree*, 8f,rub?, and Wcody Climbers of the Bombay Presidency, 2nd edition, page x, 

 Mr. W. A. Talbot has adopted the word rJiytidome. The word is believed to be 

 derived from Greek pvn$ = a wrinkle) and Zepua. ( = skin, hide) and thus signi- 

 fies wrinkled, or shrivelled, skin. pvn$ has already given rise to words which are 

 generally accepted in botanical literature, such as rytidocarpus (= wrinkled fruit), 



so that a better spelling of this useful term would appear to be rytiderm. 



a 



