142 



THE BIOLOGY OF A PLANT. 



ready been noted in the adult, p. IIT. The fibro-vascular system 

 of tissues is differentiated a little later. Different as the tissues 

 of the three systems are, it is plain from their mode of origin 

 that all are fundamentally of the same nature because of their 

 descent from the same ancestral cell; hence every cell in the 

 plant partakes more or less completely of the nature of every 

 other cell. The resemblances are primary and fundamental, the 



differences secondary and derived. 

 And what is true of the fern in this 

 respect is equally true of all other 

 many-celled organisms. 



Course of the Fibro-vascular Bundles. 



Certain features of the disposition and 

 course of the fibro-vascular bundles in the 

 embryo and in the adult may conveniently 

 be studied at this point. From the point 

 of junction of the bundles of the first leaf 

 and first root (Figs. 79, 81, 82) is developed 

 one central bundle traversing the young 

 rhizome and sending branches into the new 

 leaves and roots until 7-9 leaves have been 

 formed. After this time the rhizome 

 forks, and the course of the fibro-vascular 

 bundles in each fork is henceforwards com- 

 Ftg. 83. (After Sachs.)— Young pQ^^d. A lateral depression appears in 

 maidenhair-fern (Adiantum) at- \ ^ 



tachedtotheprothallinm,p. ?, the central bundle of each stem, rapidly 



leaf; 1, 2, the first and second increases in depth, and soon divides the 

 ^^^^^- bundle into two, one upper and one lower, 



which are best recognized in old specimens (Fig. 48). When the forked 

 shoots have reached a length of about three inches, these bundles send out 

 at a small angle towards the periphery thinner, forked branches which 

 soon unite again to form a network near the epidermis. The uppermost 

 of these branches, which passes in the median line above the axile bundles, 

 is usually somewhat more fully developed, and almost as broad as the lat- 

 ter. This structure is generally retained in the mature rhizome (Fig. 

 48, x). The number of peripheral bundles may be as great as twelve in the 

 cross-section. They anastomose in the vicinity of the place of insertion of 

 each frond, and thus form a hollow, cylindrical network, having elongated 

 meshes ; but no connecting branches between them and the two axile 

 bundles are found anywhere in the rhizome. The latter follow an en- 

 tirely isolated course within the creeping stem ; * branches from them 



* See, however, De Bary, Comp. Anat. Phanerogams and Ferns, p. 295. 

 Oxford, 1884. 



