1889-90.] On Sclerenchyuiatoiis Cells. 381" 



tributed regularly through the entire leaf-surface, and standing 

 at right angles to it, like so many pit-props. When a few of 

 these cells are detached by maceration, they remind one of 

 nothing so much as a barred gate, with both ends of the bars 

 putting out branches and forming an interlacing network. 

 Much elongated forms of these sclerenchymatous elements 

 also occur, the longest of which, from measurements made by 

 Mohl, Weisner, and others, for bast fibres, are found in the 

 Urticaceae. 



While the cell-wall itself is increasing in extent and thick- 

 ness, a further structure becomes visible — viz., the internal 

 thickening deposited in layers in the cell, termed " stratifica- 

 tion " and " striation." This is the result of a regularly alter- 

 nating distribution of water and solid material within the 

 cell-wall. Dense layers are deposited, alternating with thin- 

 ner or clearer layers. These layers are thus concentric, form- 

 ing a band nearly equal all round, and leaving a small cavity 

 or narrow canal in the centre. In other cases this thickening 

 is laid down irregularly, being broader at some points than 

 others. In one of the slides shown (sclerenchymatous cells 

 from Kaurie pine), and with the power used {\ inch), the 

 thickening or stratification appears as alternate dark and 

 light lines. 



The lignification of the sclerenchymatous elements marks, 

 along with other changes, a phase of growth in the life of a 

 plant, as it does not occur till the organs and their parts have 

 attained their mature size and form. These thickened walls 

 are perforated frequently by numerous pit-canals, in many cases 

 branched, and in others by slit-like pits. By means of these 

 pores or pits direct communication is kept up between the 

 cell-cavity and the cell-wall. Sachs, in his ' Lectures on the 

 Physiology of Plants,' holds, with regard to the ascent of 

 water in monocotyledons, that the sclerenchymatous bundle- 

 sheaths are the principal water-conducting organs. He says : 

 " In view of their considerable diameter, it seems probable 

 that the large quantity of water evaporating in the leaf-crown 

 ascends in them ; " and that, if this assumption is established 

 — which he holds as more than probable — the sclerenchy- 

 matous vascular bundle-sheaths in the stem and leaf-stalks of 

 large ferns must be regarded in the same manner. 



