GROWTH AND DIFFERENTIATION. 



141 



sented in (7, Fig. 79. A stage somewhat later than this, ^vith 

 its attacliment to the prothallium, is sliown 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 prothalHuni remains connected 

 wdth the young fern for some time, 

 and may readily be found in this 

 condition attached to ilower-j)ots in 

 hot-houses, etc. But sooner or 

 later it falls off, 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 

 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 stashes the tissue 

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

 early in development, as tlie leaf turns upwards and the root 



tt 



Fig. 81. (After Hofmeister.)— Young 

 embryo of Pteris aquiU)ia, showing 

 its attachment to the prothallium 

 by the foot ; I, leaf ; /, foot ; i\ flrsi 

 root. 



Fig. 83. (After Sachs.)— Older embryo of maidenhair-fern (AiJiautum) attached to 

 the prothallium. Seen in section. Z, 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 tlie three great systems of tissue — epidermal, fibro- 

 vascular, and fundamental. Tlie epidermal and fundamental 

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



