THE CELL WALLS OF PLANTS 



81 



are connected by numerous delicate protoplasmic strands known as 

 plasmodesms passing through fine channels in the walls. Where they are 

 relatively coarse and not too numerous, they may show plainly in 

 sections (Fig. 56), but in many tissues, particularly in meristems, their 

 extreme delicacy and the destructive effects of technics involving dehydra- 

 tion make their demonstration very difficult. For the same reasons their 

 mode of origin has not yet been clearly established. 



That plasmodesms are actually protoplasmic strands has sometimes 

 been questioned, an alternative view being that they are merely peculiar 



Fig. 56. — Soctidii i>i fiKlDsjici m i>l pci --iiriiiioii { I)i<>si)i/roii) . .showing; j)la.>iii<)<it',Miis cdii- 

 neo.ting the cells through the enormously thickened walls. (Courti.iy of General Biologiail 

 Supply HoHKc, Inc., Chicago.) 



structural features of tlie wall. Among the evidences cited in favor of 

 their protoplasmic nature aie the following: they occur only in walls 

 separating two protoplasts; when protoplasts are plasmolyzed, the}' often 

 remain connected with the wall by numerous fine strands; b^^ plasmoIysLs 

 the plasmodesms may be withdrawn from their channels; their staining 

 reactions are like those of protoplasm; they give a positive test for 

 oxj^dase; in germinating seeds the digestion of the endosperm walls 

 proceeds along the plasmodesms; after hardening the plasmodesms in 

 formalin the endosperm walls have been dissolved with sulphuric acid, 

 leaving them as connections between the undissolved protoplasts; the 



