the Stems of Succulent PlaJits. 35 



15 16 17 



Fig. 15. A longitudinal section of a branch, to show the 

 disposition of the medulla, or pith, in the ligneous centre, 

 where it forms transverse cells : the cortical cells or layers, 

 running in a vertical manner, are traversed by the spinous 

 nerve which produced the elevations in the epidermis. 



Fig. 16. represents the base of the stems of old plants. 

 The epidermis becomes hard and leather-like, and is generally 

 ruptured by the enlargement of the subspinous elevations. In 

 the case under consideration, it formed lozenge- shaped open- 

 ings round each tubercle, and retained a slight remain of the 

 impression left by the original leaf across the tubercle. 



Fig. 1 7. In this last figure decomposition had affected the 

 interior so as to enable the cortical covering to sink between 

 the stiff prominent spines, which resisted decay with greater 

 obstinacy; thus forming rhombic compartments in a reversed 

 order to wliat had taken place in the younger branches, as 

 seen in^^, 1 1 . 



A greater variety of figures might have been produced, 

 but as many were the same in character, under a few modi- 

 fications, and in progress of decay, I have chosen the most 

 decided patterns only. Some interior changes in the epi- 

 dermis, cutis, and ligneous centre presented very regular 

 markings during the progress of decomposition and desic- 

 cation. 



Fig. 1 8., taken from the inside surface of the epidermis of 

 the largest stem, will suffice to show the various changes 

 exhibited by a single plant : in this case the cortical parts, 

 under the epidermis, had separated in macerating, and left it 

 at liberty to produce this pattern, although the exterior 

 surface was not much altered from the rest of the plant. 



In this figure the hollows were numerous and deeply marked. 

 Over each a transverse, sharp, wavy, elevated line passed in 

 regular succession; undoubtedly formed after its separation 

 from the rest of the plant. These lines were not marked on 



D 2 



