MORPHOLOGY 



Pejpo in 

 section, 



ig. 89. — A, Pollen-grain of Otwurbita 

 surface view, and partly in optical 

 rendered transparent by treating with oil of 

 lemons ( x 240) ; B, part of transverse section 

 of pollen grain of Cucurbita verrucosa. ( X 540.) 



protoplasmic membrane into a cell wall. The newly-formed partition 

 wall, resulting from cell division, is developed from the cell plate, 

 which is also of cytoplasmic 

 origin. The new lamella? of a 

 cell wall in process of thicken- 

 ing are also derived from the 

 protoplasmic membrane of the 

 enclosed cytoplasm. 



The growth in thickness of 

 a cell wall by the deposition of 

 successive lamellae is termed 



GROWTH BY APPOSITION. The 



growth in surface of cell walls 

 may, in many cases, be attributed 

 to the deposition of new lamellae 

 simultaneously accompanying the 

 distension of the old. The sub- 

 sequent growth in thickness of 

 the single lamellae of the cell walls, by the interpolation of new 

 particles of cell-wall substance between the old, is designated GROWTH 



BY INTUSSUSCEPTION. 



Cell Wall Substance. — The transformation of the cell plate, or of 

 the protoplasmic membrane of the cytoplasm, into lamellae of the cell 

 wall, is -accompanied by a change in their substance. The granules 

 of the cell plate disappear and apparently dissolve, while the lamellae 

 of a cell wall are eventually formed from the solution. Possibly the 

 lamellae of cell walls possess a crystalline structure similar to that of 

 starch grains, with which they seem to correspond in many structural 

 peculiarities and in the double refraction of their layers. 



The most important constituent of cell walls is CELLULOSE. With 

 the exception of the Fungi it is present in the cell walls of all plants. 



Gilson succeeded in obtaining cellulose in a state of crystallisation. He treated 

 a plant section for a time' with cuprammonia, then washed the section carefully 

 with ammonia of a suitable concentration, and afterwards with distilled water. 

 In the cells of sections treated in this manner he found cellulose crystals in the 

 form of sphserites or dendrites. Cellulose is a carbohydrate of which the chemical 

 composition is expressed by the general formula (C 6 H 10 O 5 )n. It is insoluble in 

 either dilute acids' or alkalies. By the action of concentrated sulphuric acid it 

 is converted into dextrose. After treatment with sulphuric or phosphoric acid, 

 iodine will colour it blue ; it shows a similar reaction when exposed to the 

 simultaneous action of a concentrated solution of certain salts, such as zinc 

 chloride or aluminium chloride, and of iodine. Accordingly, chloroiodide of zinc, 

 on account of the blue colour imparted by it, is one of the most convenient tests 

 for cellulose. 



The cell walls never consist entirely of pure cellulose, but contain 

 a considerable amount of other substances, which are not stained blue 



