184 TRANSLOCATION IN PLANTS 



fine protoplasmic threads can pass through walls and thus 

 connect protoplasts with one another which were not 

 originally connected, tends to prove that protoplasm 

 actually passes through these pores, and was not merely 

 left behind as remnants or mere wall markings when the 

 parent cell divided. Such penetration through walls is 

 obvious where the connections are formed between cells 

 that were not originally in contact, as in graft unions 

 or wound healing. In fact, since there are connections on 

 all faces of a mass of cells all of which came from the same 

 parent cell, they cannot possibly have been left as remnants 

 or wall markings at the time the walls were laid between 

 dividing cells. New threads therefore must have pene- 

 trated through the walls (or new markings have developed 

 in the walls if they are merely markings as seems improb- 

 able). Examples of haustorial penetration of host plants 

 by parasitic fungi are well known and demonstrate the 

 ability of protoplasmic penetration through walls. These 

 strands, however, seem larger and more nearly the size 

 of those in sieve pores. The ready movement of certain 

 viruses from living cell to living cell and the failure of the 

 virus to enter intact living cells from the xylem, as well as 

 failure to move out into the xylem from these cells, as 

 reported by Caldwell (1931), give added evidence that 

 plasmodesma are actual protoplasmic connections which 

 allow for movement of colloidal materials, and at least 

 offer less resistance to penetration than does the surface 

 membrane. 



It seems probable that, especially in certain of the higher 

 types of sieve tubes, as in the cucurbits and vines where 

 the pores between sieve-tube segments appear large and 

 the connecting strands are relatively coarse, there is active 

 streaming of the protoplasm through the pores. It is 

 possible that strands flow in one direction through some 

 of the pores and back through other pores in the opposite 

 direction. Possibly both currents pass through a single 

 pore, for in observing protoplasmic movements one can 

 frequently see a movement in both directions along a 



