RESIN CELLS 117 



found to be composed of structural elements arranged in the 

 following order from without toward the center : (i) parenchyma 

 tracheids, (2) resin cells eventually forming an epithelium, and 

 (3) the central reservoir in the form of a cyst or canal. This 

 structure is fully exemplified in the genus Pinus, where the 

 highest form of development is attained. 



While the occurrence of resin cells in particular genera is a 

 feature of great taxonomic value, their importance in this respect 

 is greatly emphasized by the particular form of their distribution 

 and the constant tendency they exhibit toward the formation 

 of definite aggregates. In Thujopsis (plate 24) and Crypto- 

 meria (plate 26) the resin cells are always scattered through- 

 out the entire transverse section, and they show no tendency to 

 the formation of aggregates. In Podocarpus, where there is a 

 notable increase in numbers, the same general law of segregation 

 prevails, but there is nevertheless a somewhat well-defined tend- 

 ency toward aggregation. In Thuya 66.6 per cent of the species 

 show definitely scattering cells, 33.3 per cent show the cells to 

 be scattering with a tendency toward a more compact dispo- 

 sition, while in 33.3 per cent the cells fall into well-defined aggre- 

 gates or an approximation to such an arrangement. The genus 

 Sequoia is characterized chiefly by the widely scattering distri- 

 bution of the resin cells (plate 36), but in S. sempervirens there 

 are individual cases in which there is also a definite aggregation 

 into groups. In Cupressus 53.9 per cent of the species are dis- 

 tinguished by the presence of widely scattering cells, which be- 

 come definitely arranged in zones in 38.4 per cent, and aggre- 

 gated into groups in 7.7 per cent of the species. It will be 

 observed here that this feature of distribution is, on the whole, 

 more pronounced in the relatively primitive genera, and that it 

 ' diminishes in force in the genera of a relatively high order. 



In Taxodium (plate 30) and Libocedrus (plate 32), both of 

 which are distinguished by the presence of very prominent resin 

 cells, these structures are disposed in well-defined zones which 

 are concentric with the growth rings and lie either in the spring or 

 summer wood, or in both. This is to be interpreted as a definite 



