tJ 



AKATOMY AND PHYSIOLOGY OF 



Fig. 3. 



case even a greater complication of form may arise from tlie 

 growing out of particular places through unequal development^ 

 when one side of a cell lies free upon the external surface of a 

 P ^ 2 plant or in one of its internal aii^- 



*^* ' cavities, as is evident in many hair- 



structures, and in the star-shaped 

 cells of the air-cavities of the Nym- 

 phcem (fig. 2) ; but in most cases such 

 irregular growth of individual cells 

 is rendered impossible, simply by 

 the mechanical conditions in which 

 they are placed. It is a general 

 rule, that cells combined into a tis- 

 sue are bounded by a number of 

 stellate hair from thG leafstalk of :^")/m. plane surfaces, instead of possess- 

 phceaamm. j^g ^ rounded external form, since 



that part of a cell by which it is adherent to another cell becomes 

 flattened, and only the free parts of the cell-wall can follow the 

 original tendency to become rounded. The form of 

 such cells depends, therefore, principally upon their 

 relative position, and their more or less crowded 

 condition ; and the farther modification of the form 

 depends upon whether the dimensions of the cell in 

 different directions are pretty nearly equal, or one 

 dimension considerably exceeds the rest. 



Taking into consideration, in the first place, the 

 latter condition, we can divide cells combined into 

 a tissue, by no means, however,' very strictly, into 

 the short and the elongated. 



^ The short cells, developed pretty uniformly in all 

 directions, form the elements of the structure of all 

 higher plants, since all their organs are formed, in 

 their earliest stages, of these alone, and even in fall- 

 grown plants the bark and pith of the stem, as well 

 as the soft parts of the leaves and the organs of 

 fructification in general, are composed of ceUs of 

 this form. During the development of the indivi- 

 dual organs, fibrous strings are formed in the 

 cellular mass constituting their ground-work, and 

 these fibrous strings, which are composed of elon- 

 gated cells and usually also of vessels, which lie 

 among the elongated cells, receive ia this case the 

 name of vascular bundles, and taken collectively 

 constitute the wood of the plant. The mass com- 

 posed of short cells, in which the vascular bundles 

 are imbedded, is named, in contradistinction to the 

 latter, the parenchyma. 



The elongated cells of the vascular bundle 



u 



ljll>er-cell of 

 Cooos hotryophora,. 



