284 SCIENCE PROGRESS 



is still uncertain; in any case the separating portion, if 

 present, is very much thinner than the unperforated cell wall. 

 Hence it is probable that the distribution of connecting threads 

 materially affects diffusion from cell to cell. Unfortunately 

 much of the work done upon connecting threads is open to 

 question, owing to the nature of the methods employed. The 

 question of technique has been studied by Walter Gardiner 

 and by A. W. Hill and some of the results of the latter in- 

 vestigator, obtained by improved methods, may now be given. 



Hill made a critical examination of the tissues of Pinus 

 sylvestris with a view of ascertaining the nature and distribution 

 of the connecting threads. Throughout the paper, which 

 appeared in 1901, references are made to the probable value of 

 the threads as paths for the conduction of food materials. For 

 example, dealing with the threads of the endosperm, it is stated 

 that "... it is by these paths that the dissolved food materials 

 travel to the developing embryo." The arrangement of threads 

 in the cotyledons was held to indicate "... the path of 

 nutritive materials to the phloem," the albuminous cells being 

 the gateway to the sieve-tubes. In the cells of the medullary 

 rays "... the threads traversing the tangential walls . . . are 

 especially well developed, thus indicating the value of these 

 cells as passage ways through the xylem and phloem to the 

 cortex." Moreover "... the distribution of the connecting 

 threads appears to indicate the similar character of the starch 

 medullary-ray cells, the bast parenchyma and the cortical cells." 

 The assimilatory cells of the leaf are connected with the endo- 

 dermis and "... by means of these threads the elaborated 

 food material is passed in from the palisade cells." In the 

 phloem itself the connections are such that "... the albuminous 

 cells perform the important function of affording the only means 

 of communication, and this an indirect one, between the sieve- 

 tubes and the rest of the parenchymatous tissues which compose 

 the bast and medullary rays." 



Although Pfeffer has doubted the value of the threads for 

 translocation purposes, yet the work of Brown and Escombe 

 indicates that they must be of considerable importance in 

 facilitating diffusion. It was found that given certain conditions 

 ". . . the flow of diffusing substances may go on almost as rapidly 

 through a ' multi-perforate ' septum — such as that provided by a 

 pit-closing membrane studded with threads — as if no membrane 



