446 
Journal of Agricultural Research 
Vol. I, No. 6 
Only a brief discussion is given of the causes leading to 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) 1 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. EH, figs. 2 and 3; PI. 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. EH, fig. 3.) A parenchyma cell which has given' rise to 
two tyloses is shown in Plate LH, 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. EH, 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. 
