1894 THE MICROSCOPE. 21 



to represent a year's growth, but some think that these 

 rings represent only a period of growth and that a new 

 ring is added each time a new set of leaves is formed. 

 If in a season the tree's growth be stopped by drought 

 and in the fall a new set of leaves be formed the stem 

 would have two rings the width of both together being 

 a little less than that usually produced in a single year. 

 The width of the rings is usually greater near the pith 

 and decreases towards the bark. Their composition (cell- 

 ular and vascular tissue) is uniformly the same whatever 

 their thickness but the arrangement varies. Sometimes 

 the vessels predominate as in the Beech and Sycamore 

 and sometimes the cellular tissue as in the Ironwood. 

 Sometimes the vessels are uniformly distributed through- 

 out the entire ring and sometimes they predominate 

 toward the center. - Sometimes they are distributed with 

 regularity throughout the ring but the most general 

 manner of distribution is that in which most of the ves- 

 sels are near the inner surface of the ring which is the 

 first part formed. The more vessels there are, the softer 

 is the wood — Sycamore is softer than the two preceding 

 woods. Ironwood is harder than the other two because 

 it has the least number of vessels. 



SPONGES. 



By M. PFLAUM, 



pittsburg, pa. 



[Read before Iron City Microscopical Society, January 9th, 1894.] 



What is a sponge .^ Perhaps most of us might be par- 

 doned for defining it to be that soft porous toilet article 

 so useful and convenient for many purposes. We see it 

 and handle it every day, and the familiarity which in 

 this case certainly breeds indifference, deprives us of a 

 delight which the slightest study of the subject would 

 £2rive us. 



