558 SECONDARF CHANGES. 



provinces of Russia for repeatedly obtaining the suberous integuments of Betula alba. 

 In this tree also the layer of cork subsequently formed internally is distinguished from 

 that first formed by its greater softness, which depends on the predominance of delicate, 

 wide, cork-cells \ 



Sect. 178. If the woody plants investigated be compared, on the basis of the 

 data given previously, and in Sects. 175-177, with reference to the occurrence and 

 absence of the layers of periderm described, and the appearance and periodical 

 changes of the latter according to tlie time, region, and individual, the following 

 cases are found to occur. 



1, Few Dicotyledonous woody ])lants, and no Gymnosperms, keep their epi- 

 dermis, and are destitute of any formation of periderm throughout life, or during a 

 considerable number of periods of growth. Cases belonging to this category have 

 been mentioned at p. 535. Among woody plants with abundant growth in thickness 

 Acer striatum is the most remarkable known case; in stems a foot thick, at least 

 forty to fifty years old, I found the epidermis still for the most part preserved, with 

 only isolated local spots of periderm, as to which it further remains doubtful whether 

 their origin had not been caused by slight wounds. 



2. The great majority of the s/ems in question form superficial periderm; 

 a relatively small number of these (Negundo, Ilex, Sophora japonica, &c.) form 

 periderm only in the second, or a still later period of vegetation of the shoot ; it 

 is formed in the great majority during the first period of vegetation, after the ex- 

 tension and primary differentiation of tissues is complete in the internode con- 

 cerned ; in our climate the formation usually begins between the end of May 

 (iEsculus) and the end of July (Tilia). Late shoots may form periderm before 

 extension is complete^. 



Many trees confine themselves to the formation of superficial periderm during 

 life, or for many years. The periderm follows their growth in thickness. In con- 

 sequence of this they have a smooth cortex, with a suberous integument, or a 

 cracked covering of cork where the latter is massively developed (Quercus Suber). 

 This condition is permanent through life in the common Beech, and for very many 

 (nearly fifty) years at least in Abies pectinata, Carpinus, the Cork Oak, and many 

 others. 



This too, again, is an exception as compared with the majority of the cases. 

 Much the greatest number of woody plants of this category form internal periderms 

 later on in periodical repetition, and throw oflf the superficial periderm together with 

 the successive external cortical zones, in the form of scale-bark. 



As regards the age at which internal formation of periderm and desquamation 

 of the bark begin, few very accurate statements belonging to this subject exist. 

 In many trees it happens quite early ; in Ulmus elfusa as soon as the third or fourth 

 period of vegetation of the shoot, in Robinia pseudacacia, according to Hartig, often 

 even in the first. According to the same author ^ it begins in the native Oaks in the 



' Compare von Merklin, Melanges Biolog. de I'Academie de St. Petersbourg, IV. (1864), 



P- 56.V 



^ Compare Sanio, /. c. pp. 41, 58. 



- Compare the description of the trees mentioned in his Forstl. Culturpfl. 



