28 PLANT LIFE. 



pitted walls, would consist of a central mass lining the 

 cavity of the cell and giving off rays of protoplasm filling up 

 the cavities in the thickening layer ; these rays extend to 

 the outer unthickened portion of the wall, and meet similar 

 rays of protoplasm from adjoining cells. Two such rays 

 meeting each other do not usually form a continuous ray, 

 but are separated by an exceedingly thin film of wall at the 

 point of contact. 



The demonstration of the continuity of protoplasm is 

 undoubtedly one of the most important of recent discoveries 

 in plant morphology, and renders possible an intelligible 

 explanation of many phenomena in plant-life that previous 

 to this discovery appeared unfathomable. For example, the 

 communication of impulses from one part of a plant to 

 another resulting in movements, as the closing of the leaves 

 of so-called sensitive plants, or the similar movements for a 

 widely different purpose in some carnivorous plants, appears 

 to depend to a great extent on the continuity' of protoplasm ; 

 or this same arrangement renders possible in such structures 

 as sieve-tubes, where actual perforations exist, the trans- 

 ference of solid food-materials from one part of the plant to 

 another. 



Owing to a partial or entire change in the composition 

 of cell-walls, various substances of economic importance 

 are produced ; these are known as degradation-products. As 

 illustrations of such may be mentioned, cherry-tree gum, 

 gum-arabic, gum-tragacanth, along with several resins, as 

 myrrh, &c. 



Although the use of cellulose for cell-walls is almost 

 universally adopted by plants at the present day, yet there 

 is evidence to show that other substances were experimented 

 with for this purpose by some of the pioneer groups of the 



