CORRELATION OF STRUCTURE AND FUNCTION. 199 



Stricture of a Petal, and Vascular Bundle Ends. We will 

 now make ourselves acquainted with the inner structure of a 

 petal, and avail ourselves of the favourable opportunity to learn in 

 it the course and endings of the vascular bundles. The petals 

 of the common Mullein, Verbascum nigrum, readily permit us to 

 follow the branching of the bundles, and their endings, and to 

 obtain also an insight into the structure of a delicate petal. The 

 air which clings to the bright yellow petal can be easily removed 

 by tapping on the cover-glass. Alcohol should not be used, as it 

 makes the structures indistinct. The petal viewed in water shows 

 a delicate epidermis on the upper and under side, and from two to 

 four layers of spongy parenchyma. Only two layers are found at 

 the edge, from whence the thickness of the petal increases till it 

 attains four layers. The strongest vascular bundles, as well as 

 the finest branches (reduced to spiral tracheides only), are invested 

 by a layer of elongated thin-walled parenchyma. This paren- 

 chyma sheath closes together over the ends of the bundles. In 

 the cells composing it protoplasmic streaming can be seen. The 

 strongly- branched cells of the spongy parenchyma join on to the 

 elements of this parenchyma sheath. Especially instructive is 

 the view of the endings of the vascular bundles, which show a 

 radiating connection of the cells of the spongy parenchyma with 

 the sheath. 



The petals of the common Poppy, Papaver Rhceas, can also be 

 studied without further preparation, after the air has been re- 

 moved by tapping on the cover-glass. Besides the upper and 

 under epidermis, there is here present only one layer of spongy 

 parenchyma. The ends of the vascular bundles are never free ; 

 they join, on the contrary, into connected arches at the edges- of 

 the leaf. In their entire course they are surrounded by a unila- 

 mellar parenchyma sheath. To this the cells of the spongy 

 parenchyma join on from both sides. 



The petals of the Bladder Campion, Silene inflata, preserved 

 in alcohol, are remarkably transparent. The vascular bundles 

 here hardly form any network ; they fork to only a small extent, 

 but the vascular bundle endings can be readily studied. Very 

 good for this purpose are alcohol material of the standard of the 

 flower of Laburnum, and the corolla of Azalea indica. The 

 corolla of Mimulus luteus is also exceptionally good. Warming 

 in glycerine usually enhances transparency. 



Vascular Bundle Endings in Foliage Leaves. These can be 



