PATHOLOGIC PLANT ANATOMY 



361 



there was a pair of guard cells for every 204 epidermal cells. The for- 

 mation of the hairs on the edge of the ocrea of Polygonum amphibium 

 is entirely suppressed in the form natans, which is grown under water, 

 while they are present in the form terrestre. The modification of the 

 mesophyll tissue in homoplasia is due to the character of the environ- 

 ment. Plants cultivated in places saturated with moisture, or after 

 infection by fungi or animals, show a homogeneous development of 

 the mesophyll. 



In homoplasia, the vascular bundles decrease in number, the 

 mechanic tissue degenerates and the collenchyma sometimes^does not 



A B 



Fig. 142. — A, Cross-section of a normal thalloid shoot of Lunularia. {After 

 Nestler, Die natiirlicben Pflanzenfamilien I. 3, p. i?-) -B. Cross-section of a thalloid 

 shoot grown in the absence of light. {After Beauverie in Kiister Pathologische Pjianzen 

 Anatomic, 1903: 42.) 



form. Thouvenin by the use of mechanic pressure retarded the 

 development of the woody tissues in the stem of Zinnia. The stems of 

 Cardamine grown under water develop no mechanic tissue. The 

 length of the vascular bundles is less in plants grown in moist places 

 over plants which transpire strongly. Stahl found in his study of the 

 leaves of Laciuca scariola, that the mesophyll consists of palisade cells 

 throughout in the vertical leaves and in horizontal leaves lighted from 

 above of palisade cells only on the upper side of the leaf. If we call 

 upon homoplasia to explain the formation of shade leaves (Fig. 142), as 



