MUCILAGES. DEVELOPMENT OF THE FLOWER. 429 



of the seed is again very evident, and gives it its slippery 

 character. 



Forms of Miicilage. The various mucilages which are pre- 

 sent in the cells and in intercellular reservoirs of plants have been 

 classified into three groups, corresponding with the three funda- 

 mental substances which take part in the composition of vegetable 

 membranes. In this way we can distinguish cellulose, pectose 

 and callose mucilages. The cellulose mucilages coagulate in 

 a mixture of hydrochloric acid and alcohol, and then appear 

 insoluble, and do not even swell, in a solution of ammonium 

 oxalate, which dissociates the tissues. In water they swell slowly. 

 They have the general properties of cellulose, stain easily with 

 stains which colour cellulose, especially after previous treatment 

 with potash, but hardly react to iodine compounds. Cellulose 

 mucilages are rare ; an example is found in Salep, the mucilage of 

 Orchids. 



The pectose mucilages, to which the greater number of so- 

 called true mucilages belong, swell pretty quickly in water, and 

 are well nigh completely dissolved. The solution is viscous, and 

 rilters very slowly. Pectose mucilages are coagulated by acetate 

 of lead, alum and sulphate of iron, do not stain in cellulose 

 reagents, and take a yellow colour in iodine solutions. To them 

 belong the mucilages of Malvaceae, Tiliaceae, Rosaceae, the jelly of 

 many Algae, e.g., Nostoc, etc. 



Callose mucilages at first scarcely swell at all ; then, however, 

 dissolve suddenly, without the interstages which pectose muci- 

 lages show. They are found in all tissues and membranes which 

 are distinguished by rapid solution, e.g., the callus of sieve-tubes, 

 the sporangium wall of Mucorines, the cell- wall of pollen mother- 

 cells. 



With polarised light cellulose mucilage is illuminated with the 

 crossed prisms just as cellulose is ; the other mucilages are opti- 

 cally inactive. 



Mucilaginous mixtures are also found, especially of cellulose 

 and pectose mucilages. Such a mixture is found in the spermo- 

 derm of Sinapis nigra and S. alba, of Linum, etc. 



Development of the Flower. We will now complete our con- 

 ception of the nature of the floral parts by following the early 

 stages of their development. For this purpose the Cruciferous 

 liower is very well suited, and, while practically any species will 

 do, we will select the Rape, Brassica Napus. In any case an 



