124 Bulletin 79 



center of the plant are thick. Its cells are characterized by the 

 presence of crystals of calcium oxalate; when young these are star- 

 shaped, but with age they lose their beautiful stellar appearance 

 and come to occupy practically the entire space in the cell. Later 

 crystallization of these bodies through lack of space is very irregular. 

 The calcium oxalate everywhere lies closely next the inner membrane 

 of the cell and forms, with similar material of the neighboring cells 

 of the same layer, a continuous crystaUized wall under the epidermis. 

 The next layer of tissue is usually three to five cells thick and 

 with age has very thick membranes, which also are suberized, though 

 not so heavily as the cuticle. This layer is everywhere furnished 

 with plasmodesmi fi. e., connections of protoplasm between different 

 cells through the cell walls,) which are rather difficult to detect but 

 which are shown beautifully under the microscope when the sHdes 

 are immersed for a few minutes in each of the following solutions, 

 in the order named: 



1. 1 per cent osmic acid. 



2. Potassium iodid in iodine. 



3. 25 per cent sulphuric acid. 



The sulphuric acid swells the membranes and shows the plas- 

 modesmi clearly. The writer observed that tissues afterwards put 

 for a moment in Delafield's haematoxyUn until the membranes 

 become light red sometimes show good results. 



The tissue lying between the thick-celled layers, already described, 

 and the woody part of the joint, is a large-celled parenchyma, or 

 soft tissue. In comparison with the others its membranes are thin, 

 and often show plasmodesmi, which can only be detected, however, 

 with the already noted precautions. These cells are mucilaginous 

 and contain several large chloroplasts or green bodies. Kach of these 

 chloroplasts contains, during sunny days, from 4 to 12 small starch 

 grains. The cells toward the central part of the joint contain fewer 

 chloroplasts and less of the mucilaginous matter, but, on the 

 other hand, more water. Some cells of this parenchyma show star- 

 shaped crystals of calcium oxalate somewhat similar to those 

 already noted. 



The origin of the different layers and cells of a stem, or joint, 

 may be studied to best advantage in young joints. Here the 

 epidermis appears as a tissue formed by the dermatogen,* while 

 the cells of the crystal-bearing layer, the thick-celled layer, and 

 adjacent large-celled parenchyma (all together forming the cortex) 



• Dermatogen and periblem belong to the primary tisiuea of a growing stem. 



