428 LECTURE XVII. 



It is a matter of common observation that when plants are 

 grown in a window, that is, under conditions in which light 

 falls laterally upon them, their stems curve towards the light, 

 and their leaves place themselves so that the rays of light 

 entering by the window fall as nearly as possible perpen- 

 dicularly upon their upper surfaces. Confining our attention 

 to the radial organ, the stem, we see that it curves in such a 

 way as to direct its apex towards the source of light, that light, 

 in fact, induces heterauxesis. Organs which behave in this way 

 are said to be positively heliotropic, or, to use Darwin's termino- 

 logy, simply heliotropic. It may be stated that, as a general 

 rule, radial shoots and leaves using the word shoot in the 

 wide sense in which Sachs employs it are positively helio- 

 tropic. As examples we may mention primary shoots, in- 

 cluding the delicate stems of Chara and Nitella (Hofmeister), 

 many peduncles, the multicellular stipes of some Fungi such 

 as species of Coprinus (Hofmeister), and Claviceps purpurea 

 (Duchartre), the sporangiferous hyphae of unicellular Fungi 

 such as Mucor and Pilobolus (Hofmeister, Sorokin), the fila- 

 ments of unicellular Algae such as Vaucheria. Among leaves, 

 examples are afforded by the long radial leaves of various 

 Monocotyledons, such as those of the Onion. Positive helio- 

 tropism has also been observed, as an exceptional case, by 

 Hofmeister in old roots of Ranunculus aquatilis, and by 

 Wiesner in the roots of the Onion (A Ilium sativum), but it is 

 not well marked, and is only exhibited when the light is 

 intense. 



But the heterauxesis of a growing organ when light falls 

 laterally upon it is not in all cases such that its apex comes 

 to be directed towards the source of light. In some cases the 

 organ curves in the opposite direction, so that its apex is 

 directed away from the source of light. Organs which exhibit 

 this kind of curvature are said to be negatively heliotropic, or, 

 to use Darwin's terminology, apheliotropic. Negative helio- 

 tropism has been frequently observed in roots ; in most aerial 

 roots, for instance (Wiesner) ; in many ordinary roots (Wies- 

 ner) ; the root-hairs of Marchantia. Schmitz states that the 

 mycelial filaments of Rhizomorpha (Agaricus melleiis} are 



