108 BOTANY 



collenchyma (Fig. 87, c) and sclerenchyma as its special mechanical 

 tissues. The collenchyma is unlignified and very elastic, and thus 

 fitted for stretching ; it is the form of mechanical tissue suitable 

 for those parts of plants still undergoing growth in length. The 

 sclerenchymatous fibres, on the other hand, are formed after growth in 

 length has ceased, and sclereides arise even later. The elongated cells 

 of the fundamental tissue also perform a certain share of the work of 

 conduction, and are specially active in the transport of carbohydrates. 

 Secondary or waste products, resulting from chemical changes, are also 

 deposited in special cells of the fundamental tissue. Consequently 

 idioblasts (p. 83), containing crystals or rows of crystal -containing 

 cells, are often met with in the fundamental tissues, together with 

 cells, tubes, cavities, or canals containing tannin, gum, resin, ethereal 

 oils, latex, or alkaloids. Such waste products are for the most part 



Pig. 123. — Transverse section of the petiole of Nupliar hiteum. I, Intercellular air-space ; 

 i, idioblast. (x 60.) 



deposited near the surface of a plant, in order to serve as a defence 

 against destructive animals, or that they may afterwards be thrown off 

 along with the superficial tissue. Cells containing these waste products, 

 particularly crystal cells and latex tubes, are often found, therefore, 

 accompanying the phloem portion of the vascular bundles. 



The Aroideae, Nymphaeaccae, and several other plant families possess a peculiar 

 form of idioblasts, the so-called internal hairs, which project into the intercellular 

 spaces of the fundamental tissue. In the wide intercellular passages of the leaf 

 and flower stems of the Water- Lily these idioblasts are stellate in form (Fig. 123). 

 Their walls are strongly thickened, and provided with short protuberances in which 

 small crystals of calcium oxalate are deposited. 



The Distribution of the Primary Tissues 

 In the body of multicellular plants a distinction between an outer 

 small-celled and firm tissue and an inner large-celled looser tissue 



