346 SCIEXCE PROGRESS. 



as it is usually called, is thus easily proved to be of difierent 

 chemical composition trom the remainder of the tissue. 



Passing over for the present the question of the exact 

 composition of this middle lamella in the much-modified 

 cells of wood, sclerenchvma, etc., and turnino^ to the voun^ 

 tissues, consisting of so-called unchanged cellulose and 

 situated in the meristem of the apex of the axis, it is some- 

 what interesting to find that from their very commencement 

 we can find evidence that the membrane is not homoeene- 

 ous, but that whatever be its orio^inal substance somethincr 

 of the nature of a middle lamella can always be proved to 

 exist. By appropriate solvents or treatment that will be 

 described later, the thickness of the cell-membrane can be 

 split into two and the young as well as the adult cells set 

 free from each other, each retaining a complete cell-wall. 



Besides this evidence of a material differing from cellu- 

 lose forming part of the cell-wall from its early condition, 

 we may note the presence in the intercellular spaces be- 

 tween the cells of parenchyma of a substance which Russow 

 (2) and others described as "intercellular protoplasm," and 

 which sometimes appears as a delicate lining to the space 

 and sometimes takes the form of concretions of various 

 shape and bulk, occasionally filling the intercellular passage. 

 The true nature of this so-called intercellular protoplasm 

 was by several botanists, notably Gardiner (3), determined 

 to be much more allied to the substance of the cell-wall than 

 to the protoplasm, though it does not give the micro-chemi- 

 cal reactions of cellulose. 



The idea of the homogeneous nature of the cell- wall is thus 

 seen to be at variance with the facts. It is strange that it 

 should have been put forward so definitely up to quite recent 

 years, as the study of the works of the botanists of forty to 

 fifty years ago shows that there was then considerable dis- 

 cussion upon the subject. 



As early as 1825, Braconnot (4) extracted from the roots 

 of several plants, especially those of the turnip and carrot, 

 from the bulbs of the onion, and from the stems and leaves 

 of many herbaceous plants and trees, a certain feebly acid 

 substance to which he gave the name of pectic acid. He 



