i'i^ 



LECTURE XX. 



salts in an alkaline solution; whereas cane-sugar only gives a blue fluid with this 

 reagent. Inulin is still more characteristic. It is dissolved in the fluids of living 

 cells : if expressed from these, however, it is precipitated in the form of minute 

 white granules which possess a crystalline structured The crystalline nature of 

 inulin becomes much more evident, however, when tubers containing inulin are 

 allowed to lie for a long time in alcohol : there are then- formed in the tissues so- 

 called sphere-crystals — i.e. crys- 

 talline aggregates , of knobby, 

 rounded, form, which in their 

 turn consist of radially disposed 

 elements proceeding from a com- 

 mon centre. Such aggregates not 

 rarely embrace a considerable 

 extent of tissue. The sphere- 

 crystals in the dark field of the 

 polarising microscope exhibit a 

 bright cross, corresponding to 

 the crystalline structure. On 

 heating up to 50-60° C. the 

 sphere crystals are again dis- 

 solved; they are also formed 

 under the influence of frost in 

 the cells containing inulin. The 

 cellulose which occurs as a re- 

 serve-material shows no essential 

 differences from that seen else- 

 where in strongly thickened cell- 

 walls, except that it never be- 

 comes lignified. It is here to be 

 added, however, that the middle 

 lamellae of such tissues persist 

 on the dissolution of the thicken- 

 ing layers, and are not dissolved 

 with them. 



We must devote somewhat 

 more time to the consideration 

 of the starch ^ This always oc- 

 curs in the plant in the form of 

 rounded, solid, hard granules, 

 which split on the application of sufficient pressure, and which may grow from a size 

 scarcely visible even with the highest powers of the microscope to grains 0*2 mm. 

 in diameter : generally, however, the grains possess a diameter of only a few 



1 With respect to inulin cp. Sachs' Boi. Zeitung, 1864 (p. 77), and Dra-geadoTS, ' MaieriaJen zu 

 einer Monogrdphie des Inulitts' (Petersburg, 1870). 



' The chief work on starch-grains is Naegeli's extensive description in the ' Pflamen-physiolo- 

 gische Untersuchungen^ of Naegeli and Cramer, 1858. 



Fig 233.— sphere-crystals of inulin. A from an aqueous solution laid 

 aside for 2^ months. At a the action of nitric acid is commencing. B cells 

 of the root-tuber of Dahlia variabilii — a thin section after lying 24 hours in 

 90 n/o alcohol was then immersed in water C two cells with half sphere- 

 crystals which have their common centre in the middle of the intervening 

 cell-wall ; from an internode 8 mm. thick at the apex of an older plant of 

 Heiiatttkus tuierosui which had lain for some time in alcohol. D fragment 

 of a sphere-crystal. E a large sphere-crystal embracing several cells (from a 

 large piece of the tuber oi Heliantkus tuberostcs after lying some time in 

 alcohol). F inulin after evaporation of the water from a thin section of the 

 tuber oi Helianthus tuherosus (X SSo : E not so rouch magnified). 



