30 VEGETABLE ELEMENTS AND TISSUES. 



ments and tissues, very thin sections must be made 

 with a razor or thin sharp knife ; these are then to 

 be placed in a little water on a slide. As the struc- 

 tures are all minute, the distinctness with which they 

 are seen will mainly depend upon the proper thin- 

 ness of the sections. When sections of dry stems 

 are to be examined, the black margins of the air- 

 bubbles contained in the cells often render the struc- 

 ture indistinct ; these must therefore be displaced by 

 first wetting the tissue with methylated alcohol, and 

 then adding water to it in a watch-glass or on a 

 slide ; or the tissue may be soaked in warm water for 

 some hours : and this is mostly requisite in preparing 

 thin sections of dry tissues. 



Attention must also be paid to the manner in 

 which the section is made, or the direction in which 

 the portion of the plant is cut. There are three im- 

 portant directions which must be distinguished, pro- 

 ducing transverse, longitudinal, and tangential sec- 

 tions. If the cuts be made across the length of a 

 stem, for instance, the section is called transverse. 

 If the cuts be made in the direction of the length, 

 through the centre, the section is longitudinal ; and 

 if the cuts are made in a direction parallel to a line 

 running down the centre of the stem, but nearer its 

 margin, it is a tangential section. It is scarcely 

 necessary to mention that an oblique section is 

 intermediate between a transverse and a longitudinal 

 section. 



