8 THE CELL WALL 



This problem assumes increased interest with reference to the fate 

 and possible renewed-formation of the plasmodesmata of certain 

 plant cells which are believed to "slide" or push past their neigh- 

 bors during dift'erentiation (cf. iSharp 1934, pp. 15-18). The 

 function of plasmodesmata is not entirely clear but Haberlandt 

 (pp. 635-638) suggests (1) that they may be essential in trans- 

 mitting both external and internal stimuli through plant tissues, 

 and (2) that in storage tissue, such as endosperm, they may 

 be "principally concerned with trans-location." Furthermore, 

 considerable evidence has accumulated within recent years which 

 indicates that the plasmodesmata, especially those of sieve- 

 tubes, are the channels through which certain economically-im- 

 portant viruses may travel. Lasth', it is probable that the high 

 degree of correlation between the cells, tissues and organs of the 

 plant ma.y depend upon the presence of plasmodesmata. Living- 

 ston, (1935) who maintains that plasmodesmata occur in all the 

 living tissues of the tobacco plant, concludes that "the evidence 

 presented by numerous investigators indicates that actual proto- 

 plasmic connections between cells, or plasmodesmata, are gen- 

 erally present throughout all living tissues of higher plants, thus 

 establishing the organism as a definitely correlated entity of inter- 

 connecting protoplasts, instead of a community of separate cells." 

 Endosperm tissue of certain seeds provides useful material 

 for a study of the general features of plasmodesmata. Obtain a 

 prepared slide of the endosperm tissue of the persimmon (Dio- 

 spyros) and examine the section under hif/h magnification. Note 

 that the greatly thickened walls are traversed by solitary or 

 spindle-shaped groups of very delicate "lines" which are the 

 ])lasmodesmata. According to Qnisumbing (1925). the plas- 

 modesmata are ninnerous in the walls of the endosperm of D. 

 discolor and D. Ahcrnii while in 7). haki and Z). ehrnastcr "they 

 are few, restrict cd ;md gi-ouped at the walls. They occur single 

 or in groups (if two, three, foui', five oi- six, and are thicker when 

 single and usually thinner in groups. 



) ) 



III. The Gross Layers of the Cell Wall. — Differentiation and 

 maturation of most phiiit cells produce marked changes in the 

 Mi'ca and lliickncss of the cell wall. Young cells, such as those 

 of embryos and terminal meristems, possess relatively thin walls 



