﻿TRUNK STRUCTURE. 



57 



Moreover, iii such sections the bundle patterns are usually present and unfailingly 

 serve to orient the several leaf bases. In the illustrations accompanying the present 

 topic examples are shown representing various stages of regularity, passing step 



by step from the normal un- 

 disturbed spiral order seen 

 in figure 18 to an extreme of 

 disturbance in the spirals of 

 a profusely fruiting lateral 

 trunk surface like that shown 

 in figures 27 and 29. In the 

 latter case a still larger area, 

 or else a tangential section 

 cut near to the cortex, would 

 be required to make clear the 

 original leaf-base spirals. It 

 is therefore evident that in 

 the Cycadeoidese the degree 

 of regularity in the leaf-base 

 spirals is, independently of a 

 knowledge of the particular 

 stages of fructification and 

 growth represented, of no 

 value whatever in the de- 

 termination of genera or 

 species. 



The second cause of great 

 variations in the general ap- 

 pearance of the armor is the 

 particidar level at which peri- 

 derm formation has taken 

 place, as in part no doubt 

 dependent on the age of the 

 trunk at the time of fossili- 

 zation, taken together with 

 the extent to which erosion 

 has removed the ends of the 

 leaf bases. 



Thus on two sides of the 

 same trunk an entirely 

 different appearance may be 

 presented, due to the differ- 

 ence in the level of the 

 surface of preservation with 



B. Transverse section through armor, passing 2.8 cm. beneath the section shown in A. TeSpeCt tO the illSertlOll of 



the leaf bases on the cortex. 



> 



c 



^ — -O — T£s^> B ^—- 



Fig. 26. — Cycadeoidea sp. T. 758. 



Fragment of a trunk estimated to have had a diameter of from 30 to 35 cm. 



showing a characteristic growth stage. X \ • 

 A. Slightly oblique tangential section cutting armor about 2 cm. beneath its surface. 

 I-Vll, fruits in a fairly well advanced stage of growth, young seeds appearing in I, and 

 111 - VII. Nos. I -20 denote the leaf bases as much thrust aside by the emergence of 

 fruits. The high angled or left-right spirals are represented by leaf bases I ; 2, 3; 4, 5; 

 6-9; 10-13; 14-16; 19. 20, respectively. The low angled or right-left spirals are 

 represented by leaf bases 3, 5.9; 1,4.8; 13. 18; 2, 7. 12. 17; 6. 11,16,20; 10.15. 

 19; and 14. 

 Transverse section through armor, passing 2.8 cm. beneath the section shown in A, 

 with leaf bases and peduncles similarly numbered. Note the increase in the size of the 

 transverse sections of the leaf bases and the marked decrease in quantity of ramentum. 



