10 THE TBANSVERSE SECTIONS OF PETIOLES OF EUCALYPTS 



containing numerous starch granules. When the section is treated with iodine this 

 portion becomes visibly violet, showing the presence of starch in abundance. 



As the hard bast is approached the walls usually thicken considerably, and the 

 cell cavity is correspondingly diminished (Fig. 2a). The purple colouration by 

 Schulze's solution shows, however, that the cell walls are of cellulose. Here we are 

 specially concerned with that form of thick-Avalled parenchyma, known as Collen- 

 chyma, or collenchymatous parenchyma. 



Sometimes the walls are thickened at the angles, or often completely round. 

 While the epidermis is a protective tissue, this is a supporting one, serving to 

 strengthen the comparatively slender petiole. It is common in parts of plants, such 

 as the leaf-stalks, where provision has to be made for increase of length as well as for 

 strength, and so the cells must be living, and have thin places to admit the necessary 

 nutriment — they are living mechanical cells. In the old and fully-formed leaf, the 

 thickening may extend all round (as seen in Fig, 4). If the living protoplasm were 

 then to disappear, and the walls become thickened and hgnified, this tissue would 

 pass into that of the hard bast. 



The cortical tissue is excavated by large and numerous cavities, and crystals are 

 very frequent, both of which will now be specially noticed (Figs. 2 and 3). 



These cavities may be named according to their position, rather than that of 

 their contents (which has yet to be accurately determined), and so we may speak of 

 them as cortical cavities. In contrast to these cavities, which are not continuous 

 lengthwise, and so vary in their number and arrangement at different parts of the 

 petiole, there are in some species canals which are constant and continuous, and 

 from their position inside the wood curve, they may be termed central canals. 



The contents of these cavities or canals cannot be called resins, for they are 

 soluble, sometimes very readily in water ; nor can they be called oleo-resins, for the 

 petioles yield no oil on distillation. They may be spoken of as kinoid, until their 

 chemical nature is more accurately determined. 



The kinos of the Eucalypts have been, and are being, investigated by Maiden,* 

 and he finds that they are divisible into groups according to their behaviour with 

 water and spirit. These groups will be referred to in connection with classification. 



Cortical Cavities.— These cavities, usually roundish or oval in shape, occur at 

 irregular intervals, and at some little distance from the epidermis (Fig. 2). They vary 

 in size and in number. The largest is about l-80in. in diameter. They are 

 distinctly visible to the naked eye when held up to the light. The number is usually 

 from 7 to 8. 



* Ptoo. Linn. Soc, N.S.W. Vol. IV., Series 2. 



