August. 1911. 



KNOWLEDGE. 



315 



remains, with the starch grains and oil drops, may persist for 

 several years ; they occur only in the younger parts of the 

 wood, but as far inwards as the fifth or si.xth .annual ring. 



AMBROSIA FUNGI. — In continuing his interesting series 

 of papers on Ambrosia Fungi, which have already been noted 

 in " Knowledge," (May, 1910), Neger adds some further 

 details (Ber. dcutsch. hot. Gcs., 1911). Neger applies the 

 term " ambrosia " to the Fungus cells which the associated 

 insects eat, as well as to the insects themselves. The insects, 

 including various beetles, mites, and so on, bore in the bark and 

 wood of trees, and the cavities of the galls or tunnels they 

 produce are lined by the Fungus layer which produces the 

 special spherical " ambrosia " cells, serving for the food of the 

 insect. This is, doubtless, a curious case of symbiosis, 

 or mutually beneficial partnership, the insects on one hand 

 being well fed by the Fungus, while the Fungus on the other 

 hand takes advantage of the boring of the insects to gain 

 entrance to the plant tissues and also gets a better supply of 

 air opened up for it. 



In his last paper, Neger concludes that the association 

 between the boring beetles and the Fungus is an adaptation 

 on the part of the former to life in wood poor in food 

 substances. The ambrosia-seeking insects are much commoner 

 in the warm and tropical zones than in the cool temperate 

 regions, and they occur not only in standing trees but also in 

 fallen ones, which are not j-et dead. In the tropics, such 

 insects often cause great damage to valuable trees yielding 

 rubber, coftee. tea, and so on. This destructive action is not due 

 merely to the attacks of the insects themselves, but to the fact 

 that they introduce the Fungus to the conducting tissue of the 

 wood, and probably more harm is done by the parasites which 

 gain admittance in this way than by the Ambrosia Fungus on 

 which the insects feed. 



Neger has cultivated the Fungus found associated with 

 various wood-boring beetles (chiefly species of Xyleboriis) in 

 order to obtain the spores and to determine its systematic 

 position. .Apparently the majority of the Ambrosia Fungi 

 are either identical with, or closely allied to, the simple 

 Ascomycetous genus Endomyccs. 



CELLOSE AND CELLASE.— It has long been known 

 that, just as starch is converted into grape sugar (glucose) by 

 the action of the ferment diastase, so cellulose — the chief 

 component of the cell-walls of plants — is changed into glucose 

 by the action of the ferment cytase. If, however, the 

 hydrolysis of starch is incomplete, the resulting sugar is malt 

 sugar (maltose), and it has recently been found that the 

 corresponding product of incomplete hydrolysis in the case 

 of cellulose is a substance called cellose. Bertrand and 

 Holderer iBiil! Soc. Chim. France, 1910), have discovered 

 that cellose is converted into glucose by a special ferment, 

 cellase. These authors find that cellose is not acted upon by 

 such ferments as maltase, sucrase, and emulsin. This 

 discovery indicates that there are probably many more 

 ferments in plants than have hitherto been recognised, and 

 that like so many other ferment actions, the digestion of 

 cellulose takes place in several stages. 



THE ECOLOGY OF CONIFERS.— The leaves of the 

 conifers show markedly " xerophilous " structure, having thick 

 cuticle, sunken stomata, reduced leaf-surface, poorly-developed 

 air spaces, and so on, — all features characteristic of plants 

 which grow in physically or physiologically dry places, and 

 which must therefore check loss of water by transpiration. It 

 is somewhat difficult, however, to reconcile this xerophilous 

 structure with the fact that conifers often grow in places where 

 app.arently there is an abundant water supply, and it has been 

 supposed that in such cases the xerophily is a character fixed 

 by heredity. It has been suggested that the xerophilous leaf 

 structure of conifers is due to the hereditary tracheidal 

 character of the wood — that is, the presence of narrow 

 trache'ids instead of wide and open vessels — and the consequent 

 limitation of the rate of flow of the water current. But this 

 explanation breaks down, for the Larch shows a rate of flow 

 of water equal to that seen in most dicotyledonous plants. 



The assumption that a character shows hereditary fixity is 

 one which calls for independent evidence, quite apart from the 

 supposed incompatibility of structure and habitat. The 

 recent work of Groom {Ann. Bot., 1910) has shown that it is 

 fallacious to judge of the xerophily of a plant from its leaf 

 structure .alone, and that a factor of fundamental importance 

 is the total leaf area. Groom considers that the northern 

 evergreen conifers are " architectural xerophytes," in which 

 the extensive surface exposed by the evergreen leaves as a 

 whole renders it necessary for the individual leaves to be 

 xerophilous in structure. He thinks it possible that concurrent 

 increases in the assimilatory surface, and in the zerophilous 

 devices generally, increase assimilation in relation to trans- 

 piration ; but that in the absence of detailed statistics bearing 

 on the subject it is impossible to explain why the conifers 

 should have adopted the device of having a large aggregate 

 surface with a greater degree of xerophytism. 



Coulter and Chamberlain (Morphology of Gymnospcrms) 

 adopt a similar view, implying that the plant is extremely 

 plastic and that in matters of adaptation it will always come 

 to adopt that habit and structure which give the greatest 

 degree of efficiency under the given environmental conditions. 

 Coulter and Chamberlain consider that the development of the 

 small and stift" needle-like leaves or the concrescent scales of 

 conifers from their assumed broad-leaved ancestors " cannot 

 be regarded as the result of a general tendency among 

 Gymnosperms quite unrelated to conditions of living ; the leaf 

 is too variable a structure and too closely related in its work 

 to external conditions to permit such an explanation of its 

 changes." 



Compton (Neiv Pliytologist. March, 1911) criticises these 

 views, and urges that the small-leaved type is the primitive 

 one for conifers and is extremely stable. The primitiveness 

 of small leaves in conifers is bound up with Seward's well- 

 known theory that conifers arose from club-mosses 

 (Lycopodiales), but even if the ancestors of the conifers were 

 large-leaved plants, it must be admitted that the same type of 

 small-leaved conifers had persisted from later Palaeozoic times 

 to the present day. The variety in habitat of modern conifers 

 is correlated with less variety in their structure, relations of 

 leaves to stem, size of leaves, and so on, than in the case of 

 Angiosperms. The larch, with its deciduous habit and lesser 

 degree of structural protection against transpiration, but with 

 its retention of the small leaves, is a striking example of the 

 persistence of the typically coniferous form of leaf. The 

 deciduous species of swamp cypress {Ta.wdiiini) and 

 Glyptostrohiis also inhabit swampy places without modifica- 

 tion of the type of leaf form. In Pliyllocladiis the plant has 

 resorted to the device of producing flattened branches instead 

 of increasing the size of its leaves. 



It would appear, therefore, that the coniferae have small 

 power to vary the character of their leaves. The predominant 

 types — the needle-like leaves of pines, firs, larch, cedar, and 

 so on, the lineal lanceolate leaves of yew, and so on, the scale- 

 like appressed and concrescent leaves of cypress, and so on, — 

 are all closely related to each other. The leaf development is 

 in all cases small compared with that of the stem — that is, the 

 conifers are fixedly small-leaved. This appears to be con- 

 nected with the absence of lateral veining of the vascular 

 bundles in the leaf. In most cases a single or double bundle 

 runs from end to end of the leaf without branching ; and even 

 in -Araucarias and other forms, where the leaf is broader and 

 many-veined, there is not found that copious lateral branching 

 and anastomosing of the veins which is associated with the 

 free branching of so many dicotyledonous leaves. The failure 

 of the bundles to branch is to some extent compensated 

 by the development of transfusion tissue — this is also found in 

 some fossil lycopods where the same limitation of the power 

 of branching prevails, along with small leaves and often 

 xerophilous structure. 



Hence it appears that the conifers are rigidly small-leaved 

 forms, and that the power of freely adapting themselves to 

 ecological conditions is strictly limited by the lack of plasticity 

 in leaf structure. It is suggested that the lack of ability of 

 the foliar vascular system to branch with ease is one of 



