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A TEXTBOOK OF THEORETICAL BOTANY 



the cells may not be otherwise different from those of the rest of the cortex, 

 which may contain plastids but no large starch grains. This " starch sheath " 

 corresponds morphologically to an endodermis, though it will avoid con- 

 fusion if w'e do not apply this term to it, since it lacks the Casparian thickening 

 and is physiologically different. Its morphological identity with an endo- 

 dermis is shown by the fact that a starch sheath in a young stem may some- 

 times lose its starch and become thickened as an endodermis at a later stage. 



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Fig. 869. — Limim perenne. Part of a transverse 

 section through a horizontally placed stem 

 showing the starch grains in the starch sheath 

 lying on the lower walls of the cells. {After 

 Haberlandt.) 



Cases also occur where the starch sheath is dissected into separate arcs or 

 strands, divided by ordinary parenchyma, as in the Nettle and at the nodes 

 of Grasses. 



These large starch grains are movable, sinking always to the lowest side 

 of the cell vacuole, and they have been called statoliths (Fig. 869), and 

 assigned a function in connection with geotropism (see Volume III). 



In underground stems of Monocotyledons, such as ConvaUaria, there is a 

 pronounced endodermis, which may be double or even triple. In the aerial 

 stems, however, the boundary of the stele is usually a sclerotic or collen- 

 chymatous zone w^hich grades insensibly into the cortex. 



