GROWTH AND DIFFERENTIATION. 



141 



f 



sented in C, Fig. 79. A stage somewhat later than this, with 

 its attachment to the prothallium, is shown in Fig. 81. After 

 tliis the leaf grows upwards into the air, the root downwards 

 into the earth, and the young fern begins to shift for itself. 

 Eventually it reaches a condition shown in Figs. 82 and 83. 

 The prothallium remains coimected 

 with the vouno; fern for some time. 



V ~ 



and may readily be found in this 

 condition attached to flower-pots in 

 hot-houses, etc. But sooner or 

 later it falls oil, and the young fern 

 enters upon an entirely independent 

 existence. The appearance of the 

 plant and the shape of the leaf do 

 not always at first resemble those 



u 



of the adult fern ; growth is also 

 more rapid at first, several leaves 

 (7-12) being developed successively in the first year (p. 112). 

 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 



FIG. 81. (AfterHofmeister.t Young 

 embryo of Pteris aquilina, showing 

 its attachment to the prothallium 

 by the foot ; 7, leaf ; /, foot ; r, 

 root. 



FIG. 82. (After Sachs.) Older embryo of maidenhair-fern (Adiantum) attached to 

 the prothallium. Seen in section. 7, leaf; r, first root; rh, beginning of the 

 rhizome ; p, prothallium ; rz, rhizoids ; ar, archegonia. 



downwards, changes take place, which lead directly to a differ- 

 entiation into the three great systems of tissue epidermal, fibro- 

 yascular, and fundamental. The epidermal and fundamental 

 systems take on almost at once the peculiarities which have al- 



