EFFECTS OF GROWTH AS SEEN IN MOVEMENTS OF PLANT-OKGANS. 11 



trees and do not grow downwards, but turn towards the darker side. In 

 the mistletoe the hypocotyledonary axis terminates in a pestle-like 

 expansion in adaptation to the bough to which the seed adheres by 

 mucilage. This peculiar organ is prepared by heredity in advance, in 

 readiness for adhesion, just like the pads of Ampelopsis Veitchii. 



In epiphytes, as orchids and aroids, some roots cling to the stems 

 horizontally, while others descend freely and vertically to the ground ; so 

 that while shade determines the former to grow towards, and haptotropism,* 

 to cling to the stem as they elongate, gravitation affects the latter. 



The climbing roots of ivy also emerge from the less illuminated side 

 of the stem. This latter, too, is acted upon by shade, for when the stem 

 reaches the top of a wall the terminal shoot bends downwards in order to 

 grow flat upon it. As it is quite immaterial whether the ivy be growing 

 on the north or south side of the wall, the movement can only be due to a 

 preference for less illumination. Of course this is now a hereditary 

 adaptation to the production of climbing roots. Similarly do the shoots 

 of Ampelopsis turn to the shady side, as they require their adhesive pads 

 to adhere to the wall. 



Ordinary leaves place themselves at right angles to incident light. 

 This might be called diaphototropism.t The consequence is that they 

 are dorsi-ventral in structure ; but some have acquired the now 

 hereditary habit of reversing the sides, as Alstroemeria. The internal 

 structure is correspondingly reversed. It is produced by a twisting of the 

 short petiole, possibly to acquire strength, just as the ovary of Orchis, 

 which has no central placenta, acts as a strengthening pillar. 



If a naturally growing blade be partially fixed in a reversed position 

 when growing it makes a most determined effort to right itself. 



Now the orthotropic growth of ordinary stems and the axial roots of 

 germinating seedlings, was presumedly and primarily (like spores) due to 

 phototropism and skototropism respectively. 



This is well seen in growing seeds, as of mustard, suspended in water, 

 and covered above by a screen, but exposed to bright illumination from 

 below only. In this case the hypocotyls all turn downwards. 



Hereditary Influences. — Besides the influence of shade, as the 

 root grows downwards gravitation acts upon it and its effects have become 

 fixed and hereditary. Similarly growth in opposition to it, primarily due 

 to phototropism, has become fixed ; so that shoots fresh cut and laid in 

 total darkness will turn up their tips. 



That one of the elements of geotropism, i.e. " turning earthwards " is 

 skototropism is seen in young roots, say of mustard when grown in water, 

 which offers no resistance. If the glass vessel be surrounded with black 

 paper, one strip only being left uncovered for the entrance of light, all the 

 roots bend to the opposite side. Boots of cabbage grow downwards 

 though illuminated only from below. 



Mr. W. G. Smith described, some years ago, how the stolons of 

 Saxifraga sarmentosa in a pot by a window grew inwards, but as 

 they produced new rosettes, they turned towards the window, their stolons 

 growing inwards till they produced secondary rosettes and so on. He 



* A term proposed by Prof. Leo Errera, for action of tendrils, etc. 

 Dia-, in the sense of "across." 



