840 MECHANICS OF GROWTH, 



growth when its direction cuts that of the axis of growth at an angle, and the more 

 so the nearer the angle approaches a right angle. 



The positive or negative character of geotropism depends as little as that of 

 heliotropism on the morphological nature of the organ. Not only, for example, are 

 all the primary roots of the seedlings of Phanerogams positively geotropic, and most 

 secondary roots which spring from underground stems, as tubers, bulbs, or rhizomes; 

 but also many leafy lateral shoots, especially those which are destined to produce 

 rhizomes or to form new bulbs {e.g. Tulipa, Physalis, Polygonum, &c.), and even 

 foliar structures, like the cotyledonary sheaths of Allium, Phoenix, and many other 

 Monocotyledons. Among positively geotropic organs must also be included the 

 lamellae and tubes of the hymenium of Hymenomycetous Fungi. All axes which grow 

 upright (and are not bilateral), petioles, and the stipites of many Hymenomycetous 

 Fungi, exhibit, on the other hand, decidedly negative geotropism. 



The geotropism, like the heliotropism, of different organs varies in all degrees. 

 It is, for example, manifested very strongly in the primary roots and upright 

 primary stems of seedlings; much less strongly in the secondary roots and in 

 lateral branches of erect stems, &c. It appears to be the general rule that when 

 lateral shoots of the same kind spring from a vertical and therefore decidedly 

 geotropic organ, the branches of the first order are less geotropic, and the further 

 ramifications still less so the higher the order to which they belong ; the exceptions 

 to this rule may be caused by special circumstances. This gradation is very obvious 

 in roots. From the primary root or a strong root springing from the stem with 

 well-marked positive geotropism, proceed secondary roots of the first order which 

 exhibit the phenomenon much less decidedly ; and from these again secondary roots 

 of the second order which apparently are not at all geotropic, and therefore grow in all 

 directions as they may chance to originate. Geotropism, like heliotropism, does not 

 depend on the organ containing or not containing chlorophyll, nor on whether it 

 consists of masses of tissue or of a simple row of cells or of a single cell. To this 

 last category belong, for example, the positively geotropic radical hyphae of the Mu- 

 corini and the negatively geotropic sporangiophores of the same family and of 

 numerous other Mould-fungi. In the same manner the rhizoids of Char a display 

 positive, the stems negative geotropism, both consisting of unicellular segments, the 

 former destitute of chlorophyll, the latter green. Whether and how strongly an 

 organ is positively or negatively heliotropic or geotropic depends altogether on its 

 importance in the economy of the plant, and hence on its physiological functions. 



From the remarkable fact that there are organs endowed with positive and 

 negative heliotropism and geotropism, and from many similarities exhibited by the 

 two phenomena, the question presents itself whether all positively heliotropic organs 

 must not possess one description of geotropism either positive or negative, or vice 

 versa; in other words, whether the two properties do not stand in some definite 

 relation to one another. This does not however appear to be the case. Of primary 

 roots, all of which are positively geotropic, some display positive, others negative 

 heliotropism ; and again, the aerial roots of Chlorophytum, Aroideae, and Orchideae 

 display very distinct negative heliotropism, but are scarcely at all geotropic. According 

 to Schmitz the same is the case with the Rhizomorphs. There appears therefore to 

 be no necessary connection between the two phenomena. 



