HOW PLANTS GROW. 221 



ary layers, we get the markings which give their names to the 

 cells, or vessels, in which they occur. As notable instances, we 

 have the pitted cells, seen in Pine wood ; reticulated cells, seen in 

 Balsam ; spiral cells, as in Cactus and others ; armular cells, as in 

 Hyacinth, Sphagmwi, etc., or scalar if orm cells, seen in ferns. 



Then, sometimes, depositio7i of crystals is seen in outgrowths of 

 the wall, as in the calcic-oxalate crystals, found in Citron leaf-cells. 

 Another variety of marking is produced by hifiltration or Incrus- 

 tation^ where particles of foreign matter are deposited among the 

 particles of cellulose. Beautiful examples of this are seen in the 

 cell-wall of the Diatomacece, where it is transformed into an almost 

 perfect coat of silica, marked with exquisite tracings ; as, for 

 instance, Pleurosigma angulatum^ so well-known to microscopists. 



We have spoken of " vessels " more than once. A vessel is 

 formed by several cells placed end to end, their cross partitions 

 being wholly or partially absorbed, so that they come to form a 

 tube, with free communication, more or less, from end to end. If 

 the partitions be only partially absorbed — i.e., at certain points — 

 we get the perforated ones seen in what are called sieve-tubes — i.e.^ 

 tubes with sieve-like partitions, such as are found in Lime, Gourds, 

 etc. If entire absorption takes place, true vessels are formed, and 

 these, of course, are marked in a spiral, annular, or other fashion, 

 according to the markings on the cells by whose coalescence they 

 were formed. A vessel, then, is simply a modification of several 

 cells, but its formation is a most important feature in cell-life. 



We omit here all mention of other very prominent cell- 

 contents, such as starchy chlorophyll, oils, and many besides, as 

 being foreign to the scope of this paper. So far, we have looked 

 at the cell as an individual only. Now we must regard it as the 

 member of a group — i.e. — • 



{b) The Cell in Combination. 



There are two chief kinds of combinations : — i. — Cell-Fusions, 

 Here the separate cells unite to form a vessel (just described), 

 whose elements are usually indistinguishable as such. They 

 simply form a uniform zvhole of a higher type. 



2. — Tissues. Here the separate elements can be easily recog- 

 nised, and retain a certain individuality, uniting only by juxta- 

 position, not by actual fusion, and not necessarily suffering any 

 absorption of partitions. 



