446 Journal of Agricultural Research vol. i, NO. 6 



Only a brief discussion is given of the causes leading fo the formation 

 of tyloses or of their function in the living plant, since studies for this 

 purpose have already been made by other investigators. 



MORPHOLOGICAL RELATIONS OF TYLOSES IN WOODY TISSUE 

 ORIGIN AND DEVELOPMENT 



A tylose can not be considered as a distinct cell, for as a rule a cell is 

 defined as a body consisting of cell substance, cell wall, and cell nucleus. 

 With very rare exceptions (Molisch) * a tylose, as found in woody tissue, 

 is not completely surrounded by a wall and has no nucleus. It is only a 

 portion or prolongation of a wood or medullary-ray parenchyma cell. 

 (PI. UI, figs. 2 and 3 ; PL LVII, fig. 2.) Frequently more than one tylose 

 is formed from one parenchyma cell, but only one active nucleus that of 

 the parenchyma cell is present, though this may be found in one of the 

 tyloses. (PI. LII, fig. 3.) A parenchyma cell which has given rise to 

 two tyloses is shown in Plate LII, figure 2. 



The growing or arching out of tyloses has been found to follow a re- 

 duction in internal pressure or cessation in sap conduction in the large 

 vessels. When this occurs, the living parenchyma cells, which possess a 

 considerable growth potential, expand and press into the adjacent empty 

 vessel cavities. In pitted vessels this expansion is localized in the thin 

 unlignified membranes of the one-sided bordered pits which are present 

 on the dividing walls between vessels or tracheids, and parenchyma 

 (De Bary; Green; Haberlandt; Hanausek; Molisch; Rees; Russow; Sachs; 

 Strasburger; and Winckler). These membranes contain plasma and 

 therefore possess the power of growth. The internal pressure of the 

 turgid parenchyma cells, when exerted against these relatively thin spots 

 or pits, causes the pit membranes to stretch and grow by intussusception 2 

 (Green ; Molisch) . The protrusions increase gradually in size and finally 

 develop into the characteristic bladder-shaped sacs known as tyloses. An 

 open passage through the space previously occupied by the unstretched 

 closing membrane of the pit is formed in this way between the tylose and 

 the parenchyma cell. (PI. LII, fig. 2.) The contents of the tylose are 

 therefore the same as those of the parenchyma cell. 



NORMAL AND ABNORMAL TYLOSE FORMATION 



It has been shown beyond doubt that the wounding of trees through 

 cuts or bruises or at the points where branches are broken off tends to 

 stimulate tylose formation, and throughout the study this mode of 

 tylose formation has been constantly borne in mind. Generally, how- 

 ever, tyloses are not due to wounding. They are a characteristic feature 

 of the normal uninjured wood of many families of trees. Nevertheless, 



1 Bibliographic citations in parentheses refer to " Literature cited," pp. 468-469. 



2 "Intussusception" means in botany, according to Nageli, the growth of cell walls by the irregular inter- 

 position of new solid particles between those already in existence. 



