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, Phy salts, 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 hyphse 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. 
