82 PKACTICAL BOTANY. 



the Elm, cut tangential sections of the phloem or of the 

 cortical tissue, mount in water, and having found one 

 or more crystals 



(i). Run some iodine solution under the cover slip : 

 the crystal is not stained. 



(ii). Acetic acid : it is not attacked. 



(iii). Dilute nitric acid : it is more or less completely 

 dissolved. 



These reactions, coupled with what is known from 

 the analysis of ash, point to the conclusion that these 

 are crystals of calcium oxalate. 



STEM AQUATIC TYPE. 



Note the cylindrical smooth stem of the Mares-tail (Hippuris 

 vulgaris), bearing whorls of simple leaves. 



I. Cut transverse sections of an internode of the stem of 

 Hippuris vulgaris; mount in glycerine and examine with a low 

 power. Observe : 



1. A well-marked Epidermis with cuticle. Here and there are 

 to be seen radiating scale-hairs. These occur especially in the 

 axils of the leaves. 



2. Cortical parenchyma : consisting of thin-walled, chloro- 

 phyll-containing cells, with large intercellular spaces. 



3. A well-marked Bundle-sheath, with the usual characters, 

 which immediately surrounds 



4. The central Vascular Cylinder. This is composed of : 

 (a) A basis of thin- walled parenchyma, in which are 



distributed 



(6) In the central part vessels of the xylem with lignified 

 walls, 



(c) Towards the periphery elements with the characters of 

 soft bast ; the sieve quality is in this case doubtful. 



II. Cut thick transverse sections of nodes ; treat with potash, 

 mount in glycerine ; and observe, with a low power, that the 



