PARENCHYMA. 



61 



204. In the youngest state of organs short parenchyma cells 

 form the whole mass ; here they are relatively small, filled with 

 protoplasm, and have 



no intercellular spaces. 

 Later the}' are changed 

 in shape and size, may 

 have conspicuous in- 

 tercellular spaces, and 

 the protoplasm ma}- be 

 replaced, at least in 

 part, by other matters. 



205. If the cells are 

 loosely aggregated and 

 have conspicuous in- 

 tercellular spaces, the 

 tissue is called spongy 

 parenchyma. The cells 

 in such cases are apt 

 to be more or less 



branched, and in some plants assume regular stellate forms. 



206. Elongated parenchyma cells are generally more com- 

 pactly combined than the short ones. They are well seen in the 



upper part of most leaves, where 

 the}* have received the significant 

 name palisade-cells. 



207. Flattened parenchyma 

 cells are the common form in the 

 vertical plates (medullary rays) 

 which radiate from the pith to 

 the bark in woody plants. 



208. The walls of typical pa- 

 renchyma cells are thin, and may 

 be variously marked with pits, 

 especially at the points of con- 

 tact with other cells. Thicken- 

 ing threads forming reticulations 

 and spirals are not uncommon ; 

 the latter occur in the aerial 

 roots of Orchidaceae. A crum- 

 pling or folding-in of the wall is seen in some of the cells of pine 

 leaves. 



FIG. 38. Forms of parenchyma in leaf of Pyrus roiu munis. (Jacobs.) 

 FIG. 39. From pith of Sambucus nigra, showing pitted walls. (Gris.) 



