ii6 



MORPHOLOGY OF TISSUES. 



stances (gum-resin) ; in Conifcra? and Terebinthacece a clear bals-am, which hardens, on 

 exposure to the air, into a firm resin. 



The resin-passages run mostly in straight lines, or follow the course of the fibro- 

 vascular bundles ; apparently they only rarely anastomose. They so far resemble the 

 simpler l^ticiferous vessels, that they may also form continuous system's running through 

 the whole plant. When they occur in the parenchyma of the cortex and the pith which 

 is formed from the primary meristem, they are mostly distributed at nearly equal distances 

 through the transverse section of the stem, forming a circle ; when produced in phloem 

 or xylem, they may recur periodically as elements of this system, and, so to speak, be 

 formed in layers, i. e. in concentric circles, as, e, g. in the wood of Pinus, and in the 

 phloem of Coussonia. 



The occurrence of these passages is limited to certain groups ; they are found in 

 a high state of development in Coniferse and Gycadeee, Terebinthace;^, Umbelliferse, 

 Araliaceae, and Composit?e. 



B 



Fig. 97-— Transverse section of resin-passages (ff) at the base of a first year's branch of Finns syl-vestris (XSSo). A, B, C 

 passages lying in the peripherj' of the pith (s^^ spiral-vessels of a fibro-vascular bundle) ; at A the formation of a passage has 

 not taken place, but the cells destined for its formation are there, their walls having become weaker ; D wood-cells (h) enclosing 

 a group of resin-cells, not forming a passage (st a medullary ray) ; E part of the wood containing a resin-passage (g) ; next it wood- 

 cells containing starch (a}?t), fonning in the wood a zone passing in a tangential direction from one passage to another. 



When the passages lie in a tissue which undergoes rapid growth in diameter, they 

 not unfrequently attain a considerable size in this same direction ; as, e.g. in the primary 

 cortex and leaf of Pinus (Fig, 60, >6), Cycas, &c. When, on the other hand, the growth 

 of the tissue in diameter is inconsiderable, as in the wood of Pinus, the intercellular space 

 which has become enlarged into a passage remains also small (Fig. 97, B, C, g). In the 

 pith of the first-year's twigs of Pinus groups of cells are also found which resemble, in 

 contents and form, the environing cells of resin-passages, but do not separate from one 

 another, and thus do not form a passage. In this case the wood which is already formed 

 prevents a subsequent extension of the pith in diameter, and thus the space is wanting 

 which would be necessary to the formation of the intercellular space (Fig. 97, ^, D). 



