"unial!Marcri??l7o!] MicTOScopic Structure of Plants. 157 



the cells become widely separated from each other. In tissues in 

 which rapid growth takes place, we oiten find that numerous sphts 

 take place in the lamella. These splits form generally at those 

 parts where the walls of contiguous cells join, or at other times 

 the splitting is regular, the cells becoming altered in shape until 

 we have star-shaped cells formed, as in the pith of the Eush and 

 petiole of Banana. This process of splitting of the lamella may 

 go on until the cells are isolated completely, as seen in many ripe 

 fruits. It is by this splitting process that we have the intercellular 

 spaces, resin-canals, stomata, &c., formed. 



The tissues of a plant may be similar or dissimilar. We may 

 have a single ceU performing all the functions of plant life. Higher 

 up we have a tissue composed of rows of cells, as in many of the 

 lower Algae ; while in the higher plants we have numerous different 

 kinds of cells grouped together in various ways. We can at first 

 distinguish three layers of tissue in the highest plants : the external 

 or limitary tissue, the fibro-vascular bundles, and lastly the cellular 

 mass filling up the intervening space, the primitive tissue of Saclis. 

 As growth goes on, these three systems become more complex, and 

 difierent kinds of cells are formed in each. The chief forms of ceUs 

 entering into the composition of all plants are : — 1st, thin-walled 

 cells, the length not greatly exceeding the breadth, and having inter- 

 cellular spaces — Parenchyma ; and, 2nd, cells placed in rows, gene- 

 rally considerably elongated, with overlapping ends, no intercellular 

 spaces, and often gi'eatly thickened — Prosenchyma. Both of these 

 kinds of tissue may become hardened, as seen in the sclerenchyma 

 developed in cork, fruits (as the pear), &c. This hardening is, how- 

 ever, only a physiological change in the condition of the cells. 



Naegeli considers all cells as either being capable of dividing or 

 not. The first he calls Meristem, the second iDermanent tissue. The 

 cells out of which aU the parts are formed in the root, stem, &c., 

 Naegeli called primitive meristem, while the cambium cells which 

 are only found in a certain locahty he denominated secondary 

 meristem. 



Limitary Tissues. — These are only developed in plants com- 

 posed of many cells. They form a protection to the plant, and are 

 best developed in parts exposed to air and hght, less so in those 

 underground or in water. The cells of which the limitary tissues 

 are composed are generally of small size, and the walls are strong 

 and thick. In higher plants we have a layer which is in general 

 easily separable from the others, the Epidermis. Beneath the epi- 

 dermis we have the subepidermal tissue, or collenchyma. This is 

 well seen in the Begonia, and it is pecuhar in having masses of thick- 

 ening formed at the corners. Cork is also developed in the more 

 permanent hmitary tissues of the higher plants. The epidermis 

 disappears, and the cork is formed by the late-formed cells of the 



