362 GENERAL PLANT PATHOLOGY 



the unavoidable product of some arresting factor, then the structure of 

 shade leaves and those from alpine habitats, as well, as those placed under 

 water and which have a shade leaf structure, lose their remarkable char- 

 acter. Taking into consideration all of the experiments which have been 

 performed, it may be stated in concluding this chapter, that all of the 

 described hypoplasias may ' be traced back to scanty nourishment. 

 We are probably correct in assuming that there is poor nutrition in 

 plants grown in distilled water, in the dark, in an atmosphere deprived 

 of its carbon dioxide in moist places, or under water. Insufficient 

 nourishment leads to an arrestment of differentiation and this becomes 

 evident in a number of ways. 



Metaplasia 



Metaplasia has been defined as the progressive change of any cell, 

 which is not connected with cell division and cell growth. The empha- 

 sis in this definition is upon the word progressive in contradistinction to 

 the word regressive. Metaplasia is less important in the histology of 

 plants than it is in animal histology. Changes of a metaplastic kind 

 are produced in the cells of plants, especially in the production of new 

 cell contents, or of the cell wall by increase in thickness. 



Cell Contents. — Frequently, it happens with tubers, bulbs, rhizomes 

 and roots of many plants that they develop a green color in place of 

 their normal chlorophylless character. Potato tubers kept in a damp, 

 warm, sunny place sometimes develop a green color and become 

 poisonous through the formation of metaplastic solanin. Bonnier 

 found that the tissues of his experimental plants exposed to strong arc 

 lights turned green even to the pith. Likewise red pigment dissolved 

 in cell sap may appear as a metaplastic change. For example, the nor- 

 mally green pitchers of Sarracenia purpurea become purplish green when 

 the plant is grown in intense sunlight. Such is also true in the heather, 

 Calluna vulgaris, Azolla, many succulents as Opuntia and Sedum. In- 

 jury to plant parts may be followed by the development of a red color. 

 The normal color of the leaves of Saxifraga Ugulata are green, but if leaves 

 are cut through the midrib, a red coloration developed along the edges 

 of the wound. Parasitic fungi may cause a local reddening of the cells 

 affected as in certain fruit and leaves spot diseases. The metaplastic 

 formation of coloring matters appears in the so-called graft hybrids. 



