138 PRACTICAL BOTANY 



traced as follows. From one of the epidermal cells a smaller cell 

 is cut off ; from this are successively cut off the three subsidiary 

 cells : the remaining cell is the mother-cell of the stoma, which 

 divides to form the two guard-cells. 



iv. Aquatic Type. 



Note the difference in form between the submerged 

 and aerial leaves of Hippuris : the former are long, 

 very thin, especially at the margins, and of a pale green 

 colour ; the aerial leaves are shorter, more bulky, and 

 of a deep green colour. 



I. Cut transverse sections of the aerial leaves, either 

 from fresh or alcohol material : mount in glycerine, and 

 observe 



i. The oval form of the section. 



ii. The clearly defined epidermis, with a well-marked 

 cuticle m , stomata are found on both surfaces, also 

 peculiar disk-like hairs, which will here be seen cut 

 through vertically, and are inserted in the epidermis 

 by a unicellular pedicel. 



iii. The mesophyll, consisting of a lax palisade 

 tissue towards the upper surface, and still more lax 

 spongy parenchyma towards the lower. 



iv. Vascular bundles, each sharply circumscribed 

 by a bundle-sheath : one large bundle occupies the 

 centre, and smaller bundles are disposed laterally. 



II. Cut similar sections of submerged leaves, and 

 note that 



i. The section is much narrower, but more extended, 

 ii. The epidermis is clearly marked, but stomata are 



