X] 



ENDODERMIS 



187 



the observation of pitting in the radial walls, and notably in the Caspary-band itself 

 (Fig. 177, i) : partly from the fact that in many cases when the protoplasts of the endodermal 

 cells are contracted they leave the tangential walls, but remain very firmly attached to the 

 radial walls in the region of the Caspary-band. This appears to indicate a closer relation 

 of the cytoplasm to the cell-wall in that region than over the rest of the wall-surface. It 

 suggests that the endodermis may act not as individual cells, but as a living sheath, more 

 or less independent of the tissues within and without : that is, as a whole, with a common 

 coordination of its constituent cells. The character of the endodermis is assumed very early 

 in the ontogeny. In roots its first appearance coincides very nearly with the appearance of 

 the first tracheides, that is, at about the same level as the formation of the first root-hairs. 



Fig. 1 78. Series of transverse sections of the stem oiPteris {Litobrochia) podo- 

 phylla, all drawn to the same scale, showing the great increase of stelar 

 complexity as the conical stem expands upwards. { x 4.) 



In position it is first recognisable opposite the strands of phloem. Thence it spreads 

 laterally and extends along the root to its insertion upon the vascular system of the shoot. 

 In the Eusporangiatae an endodermis is always present in the root, and it is usually 

 present in the young rhizome, but it is incomplete or absent in the adult stock of the 

 Ophioglossaceae and Marattiaceae. Though present in the smaller Osmundaceae as a 

 complete investment, it is incomplete in the largest types. Often in these primitive Ferns 

 it is not restricted to one clearly differentiated layer of cells. It is not continued into the 

 leaf of Eusporangiate Ferns, though it is so in the Osmundaceae and Hymenophyllaceae, 

 which appear to take an intermediate place in this structural detail between the Euspo- 

 rangiate and the Leptosporangiate Ferns, as they do in so many other features. On the 

 other hand, in the Leptosporangiate Ferns not only does the endodermis form an unbroken 

 sheath to the vascular tissue of root and axis, but it also extends throughout the leaf. 



