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MOLECULAR FORCES IN THE I'LANT. 



starch must be heated to nearly 200' G. before its power of absorbing water materially 

 increases ; but it is then changed chemically and converted into dextrine. The cor- 

 responding action of temperature on cellulose is not yet accurately known, but it is 

 certainly different from that on starch. Like albuminoids, protoplasmic structures 

 consisting for the most part of these substances are, when saturated, coagulated. by a 

 temperature of from 50° to 60° C, while when air-dry they can stand much higher 

 temperatures without their molecular structure being destroyed \ The remarkable 

 difference in the action of temperature on saturated starch on the one hand and on 

 saturated protoplasm on the other hand must not be overlooked. In the former case 

 the power of absorbing water is enormously increased ; its structure becomes looser and 



more easily susceptible to chemical action ; 

 while the coagulation of protoplasm di- 

 minishes its power of absorbing water and 

 the diffusibility of its molecules, and in- 

 creases its power of resisting chemical 

 action. This difference is also manifest 

 when the change of molecular structure 

 is caused by acids ; and in this respect 

 normal cellulose behaves in a similar man- 

 ner to starch. 



(/6) jlcids (especially sulphuric acid) 

 when greatly diluted cause starch-grains 

 and cellulose at the ordinary temperature 

 to swell up much more violently than pure 

 water, without however destroying their 

 organisation ; and the previous condition 

 returns when the acid is washed out. If, 

 on the other hand, the acids are more 

 highly concentrated, a violent absorption 

 takes place in cellulose and starch-grains, 

 and they pass into a pasty state. Proto- 

 plasmic substances, on the contrary, co- 

 agulate, as they do under the influence of 

 higher temperatures. Concentrated sul- 

 phuric acid finally completely destroys the 

 molecular structure of both with a smaller 

 or larger amount of chemical change, and 

 they deliquesce. 



(f) Solutio}! of Potash acts on starch- 

 grains like sulphuric acid, especially in caus- 

 ing them to swell up. Its action on pro- 

 toplasmic substances is on the other hand 

 very different from that of acids ; if the 

 solution is dilute they swell up strongly or 

 deliquesce, and this is especially the case with protoplasm and the nucleus of very young 

 cells (the nuclei of older cells often resist the action strongly). But in a highly concen- 

 trated solution of potash protoplasmic structures often retain their form and apparently 

 their structure ; they neither coagulate nor deliquesce. The fundamental destruction 

 of their molecular structure which has nevertheless taken place is evident from the fact 

 that they immediately deliquesce if water is added copiously. 



Fig. 435. — Bast-cells from a leaf of Hoyn ca7->icsa (see 

 Fig. 32, p. 29) ; a and d after the coniiuencenient of the 

 action of iodine and dilute sulphuric acid ; c, wlien the 

 swelling in dilute sulphuric acid has proceded further. 

 a and ^ in a are the outermost layer not capable of swell- 

 ing, and coloured blue, which breaks up somewhat irre- 

 gularly in these cases, but in c more regularly, into a spiral 

 band, while the mner layers swell between them, and are 

 coloured blue by iodine ; 7 in r is the cavity of the bast- 

 fibfe ; 6 and i\ are constrictions at points where the outer 

 layer is especially firm ; at 6 the greatly swollen substance 

 is beginning to become disorganised (x8oo). 



"^ See Sachs, Handbuch der Experimcnlal-PhysloloL,ne. p. 6_', et seq. 



