176 



LAND- AND WATER-FORMS 



two large air-canals separated only by narrow strips of tissue. 

 Thicker outer walls and a distinct cuticle characterise the epi- 

 dermal cells of the land-form (compare Fig. 93, b and e). The 

 vascular strand of the latter has an almost continuous ring of 



lOQOQaOC 



Fig. 93. — Transverse sections of the stems of the land- («-f) and water- 

 forms {d-f) of the Starwort (Callilriclie siagnalis). a and d, entire 

 sections ; h and e, epidermis ; c and/, vascular strands, la., lacuna ; 

 Xy-, xylcm-vessels. 



xylcm (Fig. 93, c) in contrast to the one or two xylem elements 

 bordering the central canal [la.) in the water-form, whose vascular 

 strand is slightly smaller (Fig. 93, /). Similar differences are 

 encountered between the structure of the stems of the two forms 

 in other aquatics. 



