Lecture XXIII. 193 



end walls in tangential sections of the stem. These cells increase 

 in thickness in a radial direction, and after their nucleus has divided 

 a longitudinal wall is formed which divides the prism tangentially 

 into an inner and outer portion. The outer remains undifferenti- 

 ated and grows and divides, the inner one slowly changes and 

 becomes a tracheid. It elongates considerably, the originally 

 bevelled end becoming more tapered and pushing between the 

 similar ends of other developing tracheids above and below. 

 During this growth the mutual pressure of these ends distorts their 

 original form and forces them to become polygonal prisms. This 

 distortion does not take place in the middle region of the develop- 

 ing tracheid, and consequently it retains its rectangular shape 

 undistorted. Simultaneously with this elongation the developing 

 tracheid grows in width. Its growth in width appears to be 

 largely controlled by the water supply. When this is limited the 

 osmotic pressure is unable to distend the wall. Hence the tracheids 

 formed in periods of drought (usually in summer) have smaller 

 cavities than those produced when water is abundant in the stem 

 (viz. in spring). When mature the tracheids are about 2 mm. 

 in length and vary from 0-04 mm. to 0*02 mm. in width. 

 While this enlargement of the cell has been taking place the 

 protoplasm within has been active in adding to the thickness of 

 the wall. The new layers composed of woody cellulose soon so 

 increase the strength of the wall that further enlargement is im- 

 possible. Certain areas on the radial walls are not thus thickened 

 but remain as thin circular patches on the wall of the tracheid. 

 These patches are called pits. They resemble flexible membranes 

 stretched across circular openings in the radial walls between ad- 

 jacent tracheids. The central part of the membrane is slightly 

 thickened and forms a cushion of cellulose called the torus : the 

 peripheral part is excessively thin. On the margin of the pit the 

 protoplasm forms a deposit of lignified cellulose which is at first 

 a collar and then grows up into a dome overarching the pit mem- 

 brane. The dome is never completed but leaves a more or less 

 constricted opening in its top leading into the pit. Domes over- 

 arching the pit membrane separating adjacent tracheids are simul- 

 taneously developed. These structures on the walls are called 

 bordered pits. They are completed in the cell before the remains 

 of the protoplasm disappear and leave the tracheid without granular 

 contents. The bordered pits are about 0*02 mm. in diameter : 

 they are placed in a series along the radial walls of the tracheids, 

 20 to 50 on each wall. Occasionally, especially in the summer 

 tracheids, bordered pits are found on the tangential walls. The 



' 



