ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 221 



growth of plants of an extra supply of nitrogen to the roots, points to 

 the following general conclusions. The roots are enabled to form an 

 abnormally large amount of albuminoids when the nitrogen is presented 

 to them in the form of a nitrate, but only if they can obtain a plentiful 

 supply of sugar from the leaves or from reserve-food receptacles. This 

 is manifested in the greater length and thickness of the roots, in their 

 increased ramification, and in the increased amount of protoplasm in 

 their cells. 



Symbiosis.* — After a brief summary of the various known forms of 

 symbiosis, Prof. H. Marshall Ward suggests the hypothesis that one 

 syrabiont may stimulate another by excreting some substance which acts 

 as an exciting drug on the latter. One symbiont may also act on the 

 other by removing the products of metabolism, the accumulation of 

 which tends to inhibit its activity. 



Function of Aerial Roots.f — Dr. A. Nabokich has undertaken an 

 extensive series of observations for the purpose of testing the accuracy 

 of the current interpretation of the function of the aerial roots of 

 epiphytic Orchideee, Aroideae, and Liliaceae, viz. the collection and 

 absorption of atmospheric moisture. From a careful examination of the 

 structure of the aerial roots of a number of Orchideee, he has come to the 

 conclusion that the air-containing velamen can absorb only large drops 

 of dew which are brought to them by the cooling of other parts. He 

 calls special attention to two instances of the localisation of these roots, 

 viz. between the bulbs and the surrounding leaves, and in the furrow of 

 fallen leaves ; and to the occurrence of rosettes of leaves round the 

 bulbs ; as also to the fact that xerophilous plants growing on rocks, &c, 

 are not provided with aerial roots. In the main the author confirms 

 the accepted view of the function of aerial roots ; but they are very 

 often found in an active condition only on young shoots. The paren- 

 chyme of the root draws the water with great osmotic force out of the 

 velamen. In many cases, however, the velamen is replaced by several 

 layers of water-absorbing cells. One important function of the velamen 

 is to serve simply as a protection to the roots against rapid refrigeration. 

 This view is supported by the fact that the velamen is especially strongly 

 developed in those orchids which are natives of subtropical climates 

 and mountain regions. 



Physiology of Tuberous Structures.} — Herr H. Vochting finds the 

 tubers of Oxalis crassicaulis a convenient object for investigating the 

 physiological processes connected with the storing-up of food-material. 

 On germinating, the tuber produces apical buds, which develope into 

 leafy shoots, from whose base proceed abundance of roots. Later, 

 underground stolons spring from the leafy shoot, some of which de- 

 velope into other leafy shoots, while others remain underground and 

 develope new tubers at their apex. The mother-tuber yields up its 

 food-material to the daughter-tuber like the endosperm to the embryo, 

 and then perishes. A normally developed tuber has one small ring of 

 vessels. The histological structure and the physiological processes in 



* Ann. of Bot., xiii. (1899) pp. 549-62. 



t Bot. Centralbl., lxxx. (1899) pp. 331-40, 376-84, 423-32, 471-7, 503-10 (1 pi.). 



t Pringsheim's Jahrb. f. wiss. Bot., xxxiv. (1899) pp. 1-148 (5 pis. ami 9 tigs.). 



