PATHOLOGIC PLANT ANATOMY 357 



are few. The peridium, or secondary tegumentary tissue of stem or 

 root, is easily regenerated, as is seen in the formation of new cork layers 

 in the cork oak after the removal of older ones. The epidermis is not 

 always replaced but Massart found that removal of the epidermis of 

 Lysimachia vulgaris resulted in the regeneration of a new hair-bearing 

 epidermis. The regeneration of the vascular bundles has been studied 

 in monocotyledonous plants and in dicotyledons. The regeneration 

 of roots in monocotyledons consists in the replacement of epidermis, 

 phloem and xylem. In dicotyledons before the wood and bast are 

 replaced there is a regeneration of the endodermis, so that the restora- 

 tion of central cylinders, that have been destroyed, is not unusual. 



HYPOPLASIA 



The condition of hypoplasia in plants is one of arrested develop- 

 ments. The organism, or one of its parts, does not reach normal devel- 

 opment, but that development is arrested, or stopped prematurely. 

 Hypoplasia is, therefore, defective development. The plant morpholo- 

 gists and plant anatomists are chiefly concerned with the problems 

 of arrested development and recently awakened interest has been 

 taken in its study, because it has been found that the interpretation of 

 certain phenomena is subject to experimental treatment, and hence, 

 there has arisen a coterie of experimental plant morphologists. Such 

 investigators have found that the processes of growth and differentia- 

 tion are not always equally arrested, which are associated in time and 

 place in the normal course of development. For example, leaves differ 

 from the normal by their small size. They may be retarded in their 

 form, as the narrow leaves of Sagittaria produced under water, or the 

 form may remain entirely undeveloped. We will treat of hypoplasia as 

 to the number of cells, as to the size of the cells, as to the differentiation 

 of the cells and the tissues. 



A. Number oj Cells. — It has been found in a study of the dwarf 

 forms of plants such as occur on high mountain tops that the condition 

 of nanism is not so much due to a decrease in the size of the cells over 

 those of the normal plant, but is chiefly conditioned on a reduction in the 

 number of cells. The internodes of plants may be shortened, the size 

 of the leaf blade may be reduced, the thickness in the leaf may be re- 

 duced, and this reduction in size is usually associated with a loss in the 



