100 



THE BIOLOGY OF A PLANT. 



ually it reaches a condition shown in Figs. 70 and 71. The pro- 

 thallium remains connected with the young fern for some time, and 



may readily be found in this condi- 

 tion attached to flower-pots in hot- 

 houses, etc. But sooner or later it 

 falls off, and the young fern enters 

 upon an entirely independent exist- 

 ence. The appearance of the plant 

 and the shape of the leaf do not 

 always at first resemble those of the 

 adult fern ; growth is also more 



ives (7-12) 



developed successively in the 

 first year (p. 70). 

 Differentiation of the Tissues. In the earliest stages the tissue 

 is nearly or quite homogeneous, i.e., meristemic. But very early 

 in development, as the leaf turns upwards and the root down- 

 wards, changes take place, which lead directly to a differentia- 

 tion into the three great systems of tissue epidermal, fibro-vascu- 

 lar, and fundamental. The epidermal and fundamental systems 



FIG. 

 e 



>. ea. ter omeser.-oung ., . fi , pr ] 



mbryo of Pteris aquilina, showing 1 a F ST > S 



its attachment to the prothaiiium by 



the foot; 7, leaf; /, foot; r, first root. 



h. 



r, 



FIG. 70. (After Sachs.) Older embryo of maiden-hair fern (Adiantum) attached to the 

 prothaiiium. Seen, in section. I, leaf; r, first root; rh, beginning of the rhizome; p, 

 prothaiiium; rs, rhizoids; ar, archegonia. 



take on almost at once the peculiarities which have already been 

 noted in the adult, p. 75. 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 



J 



are fundamentally of the same nature because of their descent 



