20 ANATOMY OF THE GYMNOSPERMS 



The tangential section, which is also a longitudinal one, is such 

 as cuts a given radius at right angles, and it is most completely 

 such in all its parts when none of the included medullary rays 

 are cut diagonally. This result is always possible in stems of large 

 size from which typical tangential sections of several square centi- 

 meters may be cut without difficulty ; but in small stems it often 

 happens that only one or two sections of value can be obtained, 

 since the nearer the plane of section approaches the center of the 

 stem the more nearly does it approximate to a radial section. 

 It follows from this that in many cases sections will have to be 

 employed in which only a limited area exposes the typical struc- 

 ture, all the rest being partially radial. With respect to the latter 

 it should be pointed out that any deviation from a strictly tangen- 

 tial plane will involve a distortion of the structure of the medul- 

 lary ray, and inasmuch as the value of the latter for diagnostic 

 purposes rests very largely upon the form of the ray cells, it will 

 be evident that even a slight reduction of the angle from ninety 

 degrees must introduce an alteration of form which renders the 

 ray of no value. The chief value of the tangential section is 

 thus seen to lie in its exposure of the extremities of the rays, 

 the general composition and number of which may then be 

 ascertained with accuracy. From this it is possible 'to infer the 

 presence of certain structures in the tranverse section, such as 

 the resin passages, since as already pointed out there are very 

 constant relations between the occurrence of such passages in the 

 rays and in the longitudinal structure. Important exceptions to 

 this otherwise general law are to be met with in certain resin 

 cysts of traumatic origin among recent plants, and also in the 

 case of certain extinct species. Thus Sequoia Burgessii does not 

 exhibit resin passages in a transverse section, though they do 

 occur and are characteristically developed in the medullary rays. 

 Precisely similar structural conditions are to be found in Pity- 

 oxylon chasense. From the recent studies of Jeffrey, however 

 (25), we are led to the inference that such unusual relations, 

 which at first seem to indicate some peculiar feature in develop- 

 ment, may in reality be due to the fact that the longitudinal resin 



