IX VEGETABLE BIOLOGY. 239 



may be certain, therefore, that but for these microscopic pieces 

 of callus and paracallus, a few years would bring the life of our 

 trees to an end, and the wayfarer would be saddened on seeing 

 the monarchs of the forest stripped of all their glory and every- 

 where hastening to decay. 



And as with temperature, so with moisture. But for callus an 

 occasional shower or two during the dry season would stimulate 

 a tropical tree's growing-points into activity only for the young 

 shoots to perish upon renewal of the drought. Nor should the 

 agency of light pass out of view : we kuow that callus is remark- 

 ably developed in climbing-plants, and were the flow of pabulum 

 not carefully controlled, and so made available for the effective 

 organs of the plant, viz. those which have reached the light, we 

 may readily conceive how proteids and carbohydrates would 

 tend to accumulate at imperfectly illuminated points, and how 

 in consequence new and useless shoots would be formed there. 

 In short, the presence of callus and paracallus prevents the 

 expenditure of energy over the production of organs in unfa- 

 vourable situations, or under conditions which, although favour- 

 able, are only temporarily so. 



Summary. 



1. The statement made in the previous memoir to the effect 

 that Vegetable-Marrow callus gives proteid reactions, and will 

 peptonize, is an error due to the misfortune of working with 

 abnormal material. Some of the sieve-plates in the Vegetable- 

 Marrow are obliterated by true callus which neither gives proteid 

 reactions nor peptonizes; others at the end of the season are 

 blocked by the proteid body studied in the former memoir. 

 This latter substance it is proposed to call " paracallus." 



2. On the abaxial side of the bast of the Fig the sieve-plates 

 are closed-in in winter with a substance which refuses to give 

 proteid reactions and to peptonize: upon many of the inner 

 plates there is a hard proteid mass which frequently peptonizes. 

 The first substance is true callus; the second (paracallus) is 

 a further hardening of the Schleimkopf, and, when best deve- 

 loped, is characterized by taking up neither carmine nor picric 

 blue. 



3. In Ballia we find paracallus alone ; Rosa canina, Ampe- 

 lopsis tiederacea and Veitchi, the Fig, and Macrocystis pyrifera 



