ANATOMY OF STEMS. 271 



centre than near the circumference. In very young 

 stems and succulent shoots these cells are filled with 

 an aqueous fluid, and closely resemble the cellular 

 integument ; but, in older stems and twigs, they are 

 found empty, or more accurately speaking, filled with 

 air. The cells retain the hexagonal form in their emp- 

 ty state ; but in some, as in the Walnut, this is des- 

 troyed in the lamella?, into which the pith then sepa- 

 rates ; and the same occurs in the interior of the med- 

 ullary sheath of Woodbine, and similar hollow stems. 

 In the greater number of plants no vessels are per- 

 ceptible in the pith ; but in some, entire vessels con- 

 veying proper juice are present, as in the Gum-elastic 

 Fig tree, the proper juice of which is seen exuding 

 from different points of the pith in a horizontal sec- 

 tion of the stem : and in all plants, the cells commu- 

 nicate with each other by means of organized pores, 

 which are visible under the microscope. The insu- 

 lated and enclosed situation of the pith, whatever be 

 the analogies of its structure, entitle it to be consider- 

 ed as a distinct organ in every stage of its existence. 



Little is yet known with certainty concerning the 

 functions of the pith. Dr. Darwin and Sir J. E. 

 Smith considered it as important to the existence of 

 the vegetable as the column of the spinal marrow in 

 animals. Linnaeus also regarded it as the seat of vi- 

 tal energy to the plant. But Mr. Knight found that 

 on abstracting the pith from a portion of the branch 

 of a growing Vine above and below a leaf and bud, 

 " the lateral shoot, annexed, continued to live, and 

 did not appear to suffer much inconvenience ; but 

 faded a litle when the sun shone strongly on them."* 

 Indeed Caesalpinius of the 16th. century, believed the 



* Pkilos. Trans. 1801, p. S38. 



