Mr. J. D. Dana on Zoophytes. 155 



to the risk of selecting arbitrarily for measurement such rows of 

 cells as would best correspond in the form and size of their outer- 

 most wood-cells and innermost cambium-cells to a preconceived 

 theory, I measured, with the screw micrometer, in ten rows of 

 cells lying together in a radial direction, the radial diameter of 

 the two inmost cambium-cells and the two outermost wood-cells, 

 as well as the radial diameter of the cavities of the latter. To 

 extend the measurement to a greater number in the radial direc- 

 tion did not appear to me to be at all to the purpose, as the size 

 of the cambium-cells diminishes very rapidly toward the bark ; 

 on which account those cells lying further out are much less 

 suitable for comparison with the wood-cells than those cambium- 

 cells at the border of the wood which are closely approaching 

 their conversion into wood-cells. 



The average results of these measurements, expressed in frac- 

 tions of a millimetre, are as follows : — radial diameter of the 

 outer cambium-cell T ^ } the inner cambium-cell bordering the 

 wood ■£§, the outer wood-cell y 7 , the inner wood-cell -^y, the 

 cavity of the outer wood-cell TTT , and the cavity of the inner 

 wood-cell y^g. 



[To be continued.] 



XVII.— On Zoophytes. By J. D. Dana*. 



The singular features of the growing coral field, the resemblance 

 to vegetation in its productions, as well as their beauty and va- 

 riety, have long excited the attention even of those little curious 

 in the forms of living nature. Trees, shrubs, and other plants 

 of various kinds are represented with wonderful exactness, as if 

 they had been the types of this branch of the animal kingdom ; 

 and they grow mingled together often in rich profusion like the 

 plants of the land. The similarity, moreover, is not confined to 

 general form : corals have their blossoms ; for polyps are flowers 

 both in figure and beauty of colouring. Like the pink or Aster, 

 they have a star-like disc above ; and while some are minute, 

 others are half an inch or even two inches in diameter. Every 

 part of a Madrepore when alive is covered with these blossoms : 



* From Silliman's American Journal for July 1846. 



In the series of articles on Zoophytes., which it is proposed to prepare 

 for publication, the writer presents the facts and principles that have been 

 published in his Report on Zoophytes, one of the volumes of the late Ex- 

 ploring Expedition under Capt. Charles Wilkes. The subject is however 

 condensed, and the style and arrangement altered to adapt it to these pages, 

 and give it a somewhat more popular character. It is the writer's endea- 

 vour to present a succinct account of this department, about which there is 

 little generally known, without confining himself to original observations. 



