G72 



SECONDARY GROWTH IN THICKNESS 



been clone by Krah and Troschel, every series of xylem-parenchyma 

 elements is found to intersect, or to become merged in, one of the 

 medullary rays. In this way the xylem-parenchyma serves to effect 

 a connection between medullary rays situated at different levels. It is, 

 therefore, quite evident that the whole conducting parenchyma of the 

 woody cylinder forms an inter-communicating system, even though its 

 continuity may not be apparent in every transverse section. 



The relations between the conducting parenchyma (xylem-paren- 

 chyma and medullary ray tissue) and the vessels has still to be 

 discussed. In this respect, two different modes of arrangement of the 

 xylem-parenchyma must be distinguished. This tissue may take the 

 form of tangential commissures, in which case the vessels are opposed 



Fio. 27S. 



T.S. through the wood of Copai- 

 /era bracteata, as seen under a 

 simple lens, m, medullary rays ; 

 g, vessels, with externally ap- 

 posed masses of xylem-paren- 

 chyma. After Wiesner. 



Fio. 27'.. 



T.S. through the wood of Ptero- 

 carpus santaLinus, as seen under a 

 simple lens, m, medullary rays ; g, 

 vessels ; h, xylem-iiarencliyma. After 

 Wicsner. 



to, or partially embedded in these plates. This "metatracheal" type of 

 xylem-parenchyma, as Sanio calls it, occurs, for example, in Acacia 

 albicans, Ccltis trincrva, Ficus elastica, Magnolia grandiflora, Castanea 

 vesca, Juglans regia, Quercus pedunculata, etc. (Fig. 279). In other cases 

 the xylem-parenchyma envelopes the vascular tissue more or less 

 completely, without forming continuous tangential plates ; such " para- 

 tracheal " parenchyma is found in Acacia. Sophora, A. floribunda, 

 Cacsalpinia cchinata, Capparis spp., etc. (Fig. 278). Since the two 

 types of arrangement are connected by intermediate forms, and 

 may, moreover, be combined with one another in a variety of ways, 

 there is the greatest possible diversity as regards the appearance 

 presented by different woods in transverse section. Krah has attempted 

 to classify the known varieties by distinguishing a number of sub- 

 types. 



The connection between the medullary rays on the one hand, and 

 the vessels and tracheides on the other, has been made the subject 

 of special study by P. Schulz. 35: It has already been explained that the 

 medullary rays of many Abietineae are made up of two different kinds 



