PTEBIDOPHYTA 



123 



The anatomical differentiation of the sporophyte shows a 

 great advance on that of the sporogonium of the Bryophyta. 

 The epidermal and cortical or fundamental tissue systems are 



Fig. 880. 



Fig. 880. Section tbrongli protbaUium, p p, and young fern plant. K, The 

 latter attached to the former by the foot. a. Ai-chegcnia of the prothal- 

 lium. h. Root hairs of same. Afcer Saclis. 



Pig. 881. 



clearly distinguishable, and the axis is either mono-, sohizo- or 



polystelic. The stele consists of xylem and phloem, in vascular 



bundles, with circumferential pericycle in most cases. There 



is often a very marked development 



of sclerenchyma either in bands or 



patches. The leaves range from very 



simple structures to others almost as 



well differentiated as the leaves of 



Phanerogams ; but their mesophyll is 



not so well divided into palisade and 



spongy parenchyma, and the epidermal 



cells contain chloroplastids. 



The growth in length of the stem, 

 root, and leaf is usually effected by the 

 segmentation of apical cells. In some 

 cases, instead of a single apical cell 

 there is a group of them, especially in 

 the higher forms, where we have indi- 

 cated an approach to the Phanerogams. 

 Growth in thickness of the axis rarely 

 takes place ; there is, however, a modi- 

 fied cambium in Isoetes and in some 

 of the Ophioglossacece. Some of the 

 fossil forms of this group showed con- 

 siderable cambial activity, the axis be- 

 coming of very considerable diameter. 



The secondary roots are not developed from the pericycle, 

 but from the endodermis of the axis ; in the horsetails from its 



Fig. 881. Prothallium, p, p, 

 Adianlum Capillus-Vfm'ri.^ 

 seen from below, showing the 

 I'ern-plaiit developed from 

 the fertilised germ-cell of the 

 archegonium. 6. First frond. 

 «',' ?i>." Roots, h. Root hairs. 

 After Sachs. 



