Lecture XIX. 159 



isolated bands, those on the convex side alone taking up the 

 tension. Those on the concave side being very slender are only 

 to a very small degree mechanically efficient. 



The pinnae and pinnules practically owe their entire mechanical 

 support to the osmotic pressure of the substances in their vacuoles 

 acting as a compression member and to the tenacity of their cell- 

 walls and to those of the conducting strands acting as a tension 

 member. The destruction of one of these members, as in an 

 engineer's girder, leads to the collapse of the structure. This may 

 be seen by destroying the semi-permeability of the protoplasmic 

 membranes in the cells and thus rendering the osmotic pressure 

 ineffective. The easiest way of doing this is to kill the protoplasm 

 of the leaf either by surrounding it with steam or a poisonous 

 vapour. The pinnae immediately hang down in a flabby condition. 



These considerations show that the usual comparison of the veins 

 of the leaf to the ribs of an umbrella and of the soft tissue to the 

 silk is mechanically unsound. The cell-walls of the veins and 

 soft tissue being the tension members of the system are in reality 

 .mechanically equivalent to the silk of the umbrella, while the 

 osmotic pressure of the vacuoles acting on the protoplasmic lining 

 of the mesophyll cells is the compression member and corresponds 

 to the ribs of the umbrella. 



PRACTICAL WORK. 



Dig up a plant of Aspidium, shake it free from the adhering earth, and 

 sketch it about one-fifth natural size, showing the large leaves, the incurved 

 young leaves and the rough remains of the old leaves covering the stem. 

 Show some of the roots and their attachment and the tapering oldest part of 

 the stem. 



Pull off one or two of the older leaf-bases and note that each carries with 

 it two or three dark brown roots which emerge from the very base of the 

 leaves. Also notice that the conducting tracts usually break across at some 

 little distance up in the leaf-stalk and when the leaf is removed remain attached 

 to the stem. They appear as short threads hanging out of the fractured surface. 



Cut off the leaves and leaf bases as close as possible to the stem. Sketch 

 one side of the stem showing the cut surfaces of the leaf-stalks, each showing 

 five or six conducting tracts cut across as they pass into the stem. The 

 bases of the leaves are separated from one another by very dark brown grooves. 

 These grooves are lined by the superficial sclerenchyma of the stem and this 

 is the only superficial tissue of the stem exposed. 



Cut the stem longitudinally down its median axis with a strong sharp 

 knife. Put one half to steep in 10 per cent, nitric acid. Smooth the cut 

 surface of the other half with a razor. Sketch this surface about half its 

 actual size. Note the flattened apex, the incurving young leaves and leaf 

 bases of the older ones. Show the dark brown sclerenchyma forming a 

 tubular casing for the leaf bases and the whitish fundamental tissue of the 

 stem and leaf-stalks. Mark the large portions of the conducting tracts 

 exposed in the stem and the finer tracts given off from these and passing 

 outwards and upwards into the fundamental tissue of the leaf stalks. 



