FORM OF THE PLANT-CELL. 



51 



35 



Hugo Mohl was the first to point out the distinction between these 

 excavations and pores ; and daily observation confirms the existence of 

 actual cavities in the membrane. The existence of a free communication 

 between the vascular cells was early recognised ; but they were regarded 

 as originally continuous tubes, and wonderful views of their structure 

 announced, because the history of their development was not studied. 

 All vessels consist originally of vertical rows of closed cells, in which are 

 gradually deposited the secondary layers, according to the 

 forms of which they are named. When these deposit- 

 layers are tolerably perfectly formed, a process commences 

 whereby the primary cell -membrane is resorbed, so that 

 the individual cells are brought into free communication. 

 This resorption generally includes all horizontal form- 

 ations of the deposit process, but in some cases a horizontal 

 wall remains with only a pore or hole in its centre (fig. 35.). 

 Such formations present themselves very decidedly in the 

 Mosses in the group Leucophanece, as in Sphagnum, in 

 the parenchyma-cells of old Cycadece, in the so-called 

 vessels and sometimes porous cells of Coniferce, where 

 they touch the medullary rays, in the green-walled cells 

 of the root-caps of Aerides odoratum, &c. 



SECTION II. 



OF CELLS IN COMBINATION, AND INTERCELLULAR FORMATIONS. 



22. The individual cells, originating in the manner described, 

 are grouped together in various ways into great masses (called 

 tissues, tela, contextus\ which, according to their combination out 

 of various or similar elementary parts, may 

 be arranged on the following plan : 



23. A. PARENCHYMA. It forms the 

 principal mass of plants and of their parts. 

 It is, 



a. Incomplete Parenchyma, when the cells 

 barely touch each other by their parietes. 

 This may be again divided into, 



1. Spherical or elliptical Parenchyma, 

 in which the cells are round. This pre- 

 vails in succulent plants (fig. 36.). 



85 Porous vessel from Arundo Donax, in which a portion of the outer wall is removed, 

 exposing the point of union of two cells, where a horizontal wall, with a large hole in 

 it, is seen. 



K Imperfect elliptical parenchyma from the leaf of Acrostichum akicorne. The ad- 

 hering surfaces are porous. 



K 2 



