CHONDUUS. 443 



tissues become firmer and the shoot, as a whole, is there- 

 fore much strengthened in these older parts. The walls 

 of the conducting cells are very mu^h thicker and firmer 

 in the older part of a shoot than in the younger one. 



2. — Anatomy of the Root, 



The basal attachment organ, or the root part of the 

 whole plant, does not show any differentiation into the 

 three tissues met with in the shoot. It forms a flat plate 

 of tissue, from which the upright shoots arise. Its out- 

 ward form depends entirely on the nature of the sub- 

 stratum to which it is attached. It is thickest, however, 

 at the points from which the upright shoots arise, and it 

 becomes thinner towards its margin. The lower surface 

 of the attachment organ penetrates into all the numerous 

 crevices of the rock in order to firmly fix the plant. 



The cells nearest the substratum, forming what might 

 be called the " Attachment Layer," are of very varying 

 shape, and are very irregularly arranged. Their position 

 and shape depend on the varying minute nature of the 

 substratum. They have thick walls, and form a layer of 

 cells touching the surface of the rock which may be two 

 or three cells deep. But in cases where they have pene- 

 trated into and completely filled out some small hole, they 

 may form a mass of thick walled cells, connected only by 

 small but very firm strands of much elongated cells to 

 the main mass of the root (fig. 9). 



The greater mass of the root tissue proper is made up 

 of very regularly arranged rows of almost square cells, 

 which run more or less at right angles to the surface of 

 the whole attachment disc. These rows of cells are, strictly 

 speaking, always slightly curved. At the point where 

 a shoot arises they have a convex side turned towards the 

 lower end of the shoot, passing finally into and adopting 



