SECT. I MORPHOLOGY 127 



the light penetrates ; internally a colourless parenchyma is found. 

 The fundamental tissue system also takes part in jjroviding for the 

 mechanical rigidity of plants, and in connection with this function it 

 possesses collenchyma (Fig. 81 c) and sclerenchyma as its special 

 mechanical tissues. The COLLENCHYMA (p. 76) is unlignified and 

 very elastic, and thus fitted for stretching ; it is the form of mechanical 

 tissue suitable for those parts of j)lants still undergoing growth in 

 length. The sclerenchymatous fibres, which are often unlignified but 

 have greatly thickened Avails, on the other hand, are formed after 

 growth in length has ceased, and sclereides (p. 80) arise even later. 

 The elongated cells of the fundamental tissue also perform a certain 

 share of the work of conduction, and serve for 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. 80), 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 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, accompanying the phloem portion of the vascular 

 bundles. 



The Aroideae, Nyniphaeaceae, and several other plant families possess a 

 peculiar form of idioblast, in the so-called internal hairs, which project into the 

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

 of the petioles and flower stems of tlie Water-Lily these idioblasts are stellate in 

 form. Their walls are strongly tliickened, and provided with short protuberances 

 in which small crystals of calcium oxlate are dejjosited. 



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 

 soon becomes apparent. The outer tissues are best adapted for pro- 

 tection, the inner for conduction and storage. The cells of the inner 

 tissues accordingly become elongated for the purpose of conduction. 

 The outer tissues in plants, which must provide independently for their 

 own nourishment, contain chromatophores fitted for assimilation, and 

 are correspondingly coloured, while the inner tissues remain colourless. 

 The outer portion of the fundamental tissue thus differentiated is 

 called the cortex, the inner the medulla or pith. An epidermis, 

 distinguishable from the cortex, is found in some of the Mosses, but a 

 sharp distinction between these tissues is first found in the more 

 highly oi'ganised plants. 



