1902] 



CHANGE OF FORM IN PROSERPINACA PALUSTRIS 



95 



grown in the air the leaves are lanceolate in outline, 5 to 5-5^™ 

 long and 7 to 8 ""^ broad, with margins rather finely serrated 

 {^figs, I and j). The structure is that common to air leaves — 

 that is, thick cuticle, abundance of stomata on the under surface, 

 strong development of pali- 

 sades, and a well-developed 



vascular system {fig^j). In 

 the stem there are the usual 

 vascular and mechanical tis- 

 sues, cortex, etc. The whole 

 form and structure is that of 

 a typical aerial shoot. 



When submerged the 

 shoots that develop are en- 

 tirely different. The leaf is 

 finely dissected, consisting 

 of a slender central rib, with 



from three to five thread-like 

 lob 



es on each side {Jigs, 2, 

 4, and S), which are often 

 only about 0.2 ^"^ in diameter, 



and may become 2 



cm 



long. 



These filamentous divisions 

 are cylindrical in cross-sec- 

 tion, and show^ almost an en- 



FiG. 2.— Water form in nutrient solution. 



tire absence of stomata; the 

 epidermal cells are very small 

 and without any trace of cuticle, the outer walls being thin and 



{fig 



There is an absence of any sort of differen- 



tiation in the mesophyll, no trace of palisades appearing. The 

 chlorophyll has a marked peripheral placement, the outer cells, 

 which are spherical, being packed full, while the inner ones have 

 almost none. The mesophyll is very loosely arranged, and with 



pious development of air chambers throughout. The dry 

 weight is just one-tenth that of the air leaf. There is an entire 

 absence of any sort of mechanical tissue or of xylem, both of 



a CO 



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