24 



Vicia faba L. 



The first appearance of the cavity in Vicia faba is in the 3rd 

 epicotyledonary internode when the plant is but 3 or 4 internodes 

 in height. From the 3rd internode the cavity -formation progresses 

 upward and downward, being always found in the internodes near 

 the apex before they have elongated, but not appearing in the hypo- 

 cotyl till the plant is more than half grown. Thus the cavity is not 

 present in the hypocotyl till long after elongation has ended, appears 

 in the 2nd epicotyledonary internode when elongation is nearly com- 

 plete, and in all the other internodes long before extension ceases. 



The phenomena of cavity-formation during primary growth are 

 these : The pith first shows ordinary intercellular spaces which ra- 

 pidly enlarge giving a central spongy parenchyma. The intercellular 

 spaces run together forming in cross-section several large openings 

 separated by chains of cells. As these schizogenous spaces still far- 

 ther enlarge , several cells are left with but slight connection with 

 the neighboring tissue, and these cells are the first to die. The 

 several spaces a little later, both schizogenously and lysigenously, 

 unite to form one cavity which then constantly enlarges by the 

 extension of the outer rows of pith-cells and the stretching and 

 eventual death of the inner rows of pith -cells, the bundle -ring 

 meanwhile moving farther and farther from the centre. 



Etiolation does not prevent the formation of cavity; it produces 

 merely a narrower cavity. Four plants were grown in darkness with 

 the result that the cavity -formation appeared in the usual way in 

 each. One plant had a wrapping of black cloth placed about the 

 3rd internode and so grew for 46 days. Examination at the end of 

 this period failed to show structural differences induced by the 

 opaque wrapping. Two plants however that had casts about the 3rd 

 internodes and also grew in darkness showed the same differences 

 within the casts as those grown in light and later to be described. 

 Etiolation here as generally brings a weaker development of mechanical 

 tissue, but causes the disappearance of no one kind of mechanical 

 tissue. 



Experiments with Gypsum Casts. 



In the experiments on Vicia faba , casts were applied to more 

 than 50 plants, and about an equal number of control plants have 

 been used. Faba suffers very little from the cast, plants in casts 



