1 8 ANATOMY OF THE GYMNOSPERMS 



regional distribution, although to a limited extent it is true that 

 certain aspects of these elements may lead to correct inferences 

 as to their structural features when displayed in other planes of 

 section, whereby approximately correct data may be obtained 

 even in the absence of longitudinal sections. This is particularly 

 true with respect to the distribution of the resin passages, the 

 relations of their longitudinal and radial distribution being such 

 that where the one occurs the other may be inferred ; hence, if 

 resin passages occur in the transverse section, we may conclude 

 with certainty that they are also to be found in certain of the 

 medullary rays, which will also present a fusiform aspect in tan- 

 gential section. 1 The general character of the medullary ray is 

 also displayed in this plane of section, but in its least important 

 aspect. Although all the details of width and composition may 

 be obtained much more accurately, and in some cases only from 

 longitudinal sections, nevertheless the absence of these latter 

 enables us to determine from the transverse section whether the 

 rays are one or more seriate and if some of them contain resin 

 passages. 



In order that the features thus indicated may be exhibited in 

 their typical aspects, it is of the greatest importance that the 

 plane of section be exactly at right angles to the axis of growth, 

 otherwise the distortion of structure which necessarily results 

 will not only make observation difficult, but it will serve to 

 seriously impair the accuracy and value of any diagnosis which 

 may be drawn. 



The radial section is also a longitudinal section the plane of 

 which should exactly coincide with the radius of the stem. Any 

 deviation from this position will cause the section to become 

 more or less tangential, and just in proportion as it approaches 

 this latter will its value diminish. In stems or branches more 

 than five centimeters in diameter there should not be the least 

 difficulty in securing the desired result, inasmuch as the abun- 

 dance of material will admit of somewhat reckless cutting ; but 

 in small branches of one centimeter or less, such as one must 

 1 Certain exceptions to this law are to be met with in the case of fossil species. 



