48 THE TISSUES OF PLANTS 



may for the most part be obtained by very simple processes. 

 Thus if water be added to flour in small quantities, a duc- 

 tile paste will be formed which, when kneaded by the 

 hand and washed by a slender stream of water, becomes 

 a grey, tenacious, and highly elastic substance termed gluten. 

 The water employed in this process is rendered turbid and 

 milky, and a white matter remains suspended in it, which 

 is starch, as may be easily ascertained by testing it with 

 tincture of iodine. So, again, when meat has been boiled 

 for some time in water, the oil is observed to separate 

 and float on its surface ; but another substance, separated 

 from the meat, remains suspended in the water, which soli- 

 difies on cooling. This substance is termed gelatin. The 

 tasteless shreds which remain are fibrin. Now gluten and 

 starch are instances of the proximate principles of plants; 

 oil, gelatin, and fibrin are examples of the proximate prin- 

 ciples of animals. 



The walls of the cells or elementary parts of the vegetable 

 tissues, are not formed of a simple homogeneous substance, 

 but are made up of two layers of very different composition 

 and properties. The innermost layer, which is the mem- 

 brane first generated over the nucleus, has for this reason 

 been called the primordial utricle. It is very thin and deli- 

 cate, and escapes attention so long as it remains in contact 

 with the outermost layer, from which, however, it is easily 

 detached by tincture of iodine. To the primordial utricle, 

 all the subsequent vital operations are to be referred. The 

 external layer though commonly regarded as the real cell- 

 wall, is in reality a deposit of cellulose which is generated 

 on the outer surface of the primordial utricle. This is usu- 

 ally thick and strong compared with the other, and pos- 

 sesses various degrees of consolidation, from the condition of 

 mere mucus, to a firm, tenacious, and elastic substance. 



