124 



ORGANOGRAPHY. 



Fig. 236. consequence of this decaying at its 



lower pai't. 



In the Screw-pine the extremi- 

 ties of these roots, while still sus- 

 pended in the air, are sometimes 

 ]jrovided with a little cap-like co- 

 vering derived from the parenchyma 

 of the bark, which they appear to 

 have jmshed before them in the 

 course of their passage through it. 

 Such cap-like coverings may be also 

 seen occasionally upon the extre- 

 mities of other adventitious roots. 

 This calyptra is considered by ma- 

 ny to be of a ditFerent nature to 

 that which occurs on the extre- 

 mities of some other adventitious 

 roots Avhich grow in water, as those 

 of the Duckweed {fig. 236 j, Pistia, 

 and some other alhed jdants. The 

 latter is called the pileorhiza, and 

 Fig. 236. Magnified plants of the consists of a ccllular laver which 

 i^^%he'':^L'SrTd'tra ^^^ separated entirely from the tis- 

 sheath. sue of the root beneath, except at 



its extreme point, where it is in direct connexion witli it. This 

 cap remains under natural circumstances throughout the life of 

 the root, which at once perislies if it be destroyed. 



Fig. 237. 



Fig. 237. Orchideous Plants, to show their mode of growth. 



Epiphytes or Air-plants.— 1\\ these i)lants none but aerial roots 

 arc produced, and as these never reach the soil they cannot 



