PATHOLOGIC PLANT ANATOMY 359 



vated Euaslrum verrucosum, a desmidiaceous alga, in lo per cent, cane 

 sugar. The daughter cells formed by a previous division of those 

 cells divided again before they had attained their normal size. The 

 conditions in the higher plants where hypoplasia is shown by the 

 production of abnormally small cells are such that the period of elon- 

 gation, which normally follows the last cell division, does not take 

 place, or is stopped part way. Abnormally narrow tracheal tubes are 

 found in dwarfs, in etiolated and poorly nourished plants, or in in- 

 dividuals infected by fungi, or gall-producing animals. Disturbances 

 in nutrition reduce the size of the wood elements produced by cambial 

 activity. 



In the study of the differentiation of cells and tissues, those cases 

 should be considered first which concern the individual cells, where the 

 formative process may stop prematurely. An investigation of Udotea 

 Desfontainii shows the arresting action of unfavorable life conditions 

 upon the development of the cell form. The leaf-like part of this alga 

 is composed of elongated sacs, which run lengthwise and parallel, with 

 numerous side branches of limited growth, which interlock to give 

 the thallus its characteristic firmness. If artificially cultivated, the 

 parallel sacs show undiminished growth activity, but the side branches 

 no longer show limited growth, but unlimited, and the thallus loses its 

 wonted form. 



Arrestment of the development of the cell wall is indicated in the par- 

 tial, or entire cessation of the secondary growth in thickness, and as a 

 result, the elements normally thick-walled have walls of only moder- 

 ate thickness. Weak, or insufficient, transpiration acts pari passu in a 

 poor development of the cuticle of epidermal cells. Dwarfed plants 

 frequently show weakly developed cell membranes, as a sign of disturb- 

 ances in the nutritive processes. Chemic changes may be associated 

 with hypoplasia. Lignification is rarely excluded in the formation 

 under disturbing influences of the woody elements of plants. The cells 

 of the medullary parenchyma in thorns {CratcBgus) remain unhgnified, 

 when infected with a rust fungus, Roestelia. Finally, the formation of 

 cross walls may remain incomplete, thus giving rise to chambers, 

 sometimes communicating with each other. 



Hypoplasia, as it affects the cell contents, may be seen in the 

 reduction in the number of chloroplasts in variegated leaves, in plants 

 with pale-green leaves and in plants which grow in places saturated with 



