200 



XV. THE LEAF VASCULAR BUNDLE ENDINGS. 



well studied in most cases of leaves in which the texture is 

 neither leathery nor succulent. We w r ill select for the purpose a 

 Balsam Impatiens parviflora, 1 a not uncommon weed of large 

 gardens in many parts of the country. For general information 

 fresh material, when available, will serve ; for more careful study 

 of the bundle endings material hardened and decolorised in absolute 

 alcohol is preferable ; and suitable sized pieces then laid in a mix- 

 ture of three parts turpentine and one part creosote, or in a mixture 

 of creosote and alcohol, or in pure carbolic acid, or, and this is 

 best of all, in a solution of eight parts chloral hydrate in five parts 

 water. The leaf soon becomes so transparent that we can obtain 

 any desired optical section of it. We lay the piece of leaf with 

 the under side uppermost, and see first the epidermis, of very 

 sinuous cells and with stomata ; then a very wide meshed spongy 

 parenchyma ; then the palisade cells, round in optical section ; 

 then the epidermis of the upper side, like that of the under, but 

 devoid of stomata. The palisade layer is very rich in chlorophyll 



grains, decolorised by the alcohol, 

 while the spongy parenchyma con- 

 tains but few. Here and there 

 in the spongy parenchyma are 

 found long spindle - shaped cells, 

 each filled with a highly refractive 

 mass of slime, in which a bundle 

 of raphides lies. In young leaves 

 these can be seen in course of de- 

 velopment. The leaf is traversed 

 by a mid-rib, from which proceed 

 stronger and weaker lateral veins. 

 The mid-rib ends at the leaf-apex ; 

 the stronger lateral veins of the 

 first order proceed to the edge of 

 the leaf, and there join on in 

 arches to the next higher. From 

 the lateral veins of the first order 

 proceed lateral veins of the second 

 order, and these proceed to successive degrees of branching. The 

 higher the degree of branching the thinner the vein, which ulti- 



1 The transverse section of the petiole of this leaf shows a remarkable 

 gutter-like " starch sheath," which can be followed into every branching 

 vein. [ED.] 



FIG. 78. Eudiug of a vascular bun 

 die in fmpatiens jmrvi/lorci (x 240). 



