28 PLANT STRUCTURE AND GROWTH. [CHAP. 



posited in the form of a spiral thread, and these are 

 called spiral vessels (fig. 31). Another method in 

 which the thickening takes place is in the form of 

 rings, and such cells are known as anmilar vessels 

 (fig- 3 2 )- I" others, again, which are four or five 

 sided, the new deposits take place in the form of 

 thin bars across the sides, and as they thus bear a 

 resemblance to the "rounds" of a ladder, they are 

 termed scalariform vessels (fig. 33). 



All these tissues have a definite place in the struc- 

 ture (excepting the lowest forms) of plants. No 

 matter how they may be arranged 

 inside a plant, the outside of it is 

 always invested by a layer of flat, 

 close-fitting, colourless cells, called 

 i the Epidermis. Dotted over this 

 epidermis are a number of little 

 holes, each surrounded by two kid- 

 ney-shaped cells (fig. 34). These 

 orifices are termed Stomates^ or mouths, because they 

 are used for breathing purposes. Each stomate com- 

 municates with the intercellular spaces we spoke 

 about just now, and through them carbonic acid gas 

 is absorbed from the atmosphere. 



This is the internal structure of a plant ; with its 

 external characters, we suspect, most of our readers 

 are tolerably familiar, yet for those who are not, we 

 will give a brief account of it. 



Fig- 35 represents the ideal plant. We observe 

 that it consists of a main stem, more or less branched 

 at its two ends. The branches at the upper portion 

 of the stem bear leaves and floivers, those at the 



FIG. 



