THE LIFE-HISTOEY OF PLAXTS. 



299 



thick ; in a tuber it is usually more or less elon- 

 gated. It is, however, rather in their production at 

 the ends of longish thread-like branches that the 

 difference mainly consists. Tubers are generally 

 grouped by gardeners as forms of roots; but this 

 arrangement by no means pleases the botanist, and 

 is, indeed, fatal to clearness of conception, while the 

 treatment that is appropriate to a tuber is by no 

 means suitable to a root. The technical difference, 

 not to mention differences of structure and mode of 

 growth, consists in the fact that in 

 a tuber there are eyes or buds, while 

 a true root is destitute of these pro- 

 ductions. Xevertheless, there are, 

 as has been already mentioned, some 

 awkward intei-mediate formations, 

 the plague of the botanist, the des- 

 pair of the devotee of exact science, 

 which will not fit in comfortably 

 A\-ith any definition that may be 

 framed ; such are the tubers of the 

 Dahlia, of terrestrial Orchids, the 

 thick stem of the Cyclamen, or the 

 pseudo - bulb of the epiphytal * 

 Orchids, and many others. How- 

 ever different morphologically — 

 that is, structurally — they are 

 but illustrations of nature's method 

 of compassing the same ends by 

 varied agents. They are very in- 

 teresting to the morphologist and 

 physiologist, but of little \a.hie to 

 the systematic botanist, because 

 they afford little or no indications 

 of real affinity, plants of the most 

 diverse lineage ha^TUg in this 

 matter similar structure. The most 

 useful course appears to be to allude 

 to such of these formations as are 

 most important to the cultivator, 

 and to do so from his point of ^-iew and that of the 

 physiologist. So considered, all these nondescript 

 structures fall very naturally and conveniently 

 under the head of Eeserve-organs, taking their place 

 as such by the side of seeds, buds, bulbs, and other 

 organs of storage, and illustrations of intermittent 

 energy' or suspended animation. 



First and foremost in importance is the Potato, 

 " the noble tuber." Great as is its importance to us, 

 that importance is largely artificial. Of the seven 

 or eight hundred species of SoJanum known in a 

 wild state, only some half-dozen are known to pro- 

 duce tubers, and only one, S. tuherosum, the Potato 



* Epipli}-tal, a term applied to plants which, grow upon 

 other trees, hiit -n-hich, as they do not penetrate beneath 

 their surface, do not derive any nourishment from them. 



par excellence, has been brought into cultivation. So 

 far, all the numerous varieties of Potato in cultiva- 

 tion are mere seedling or selected variations from 

 this one species. The point we would here impress 

 is that the formation of tubers so important to us 

 is clearly of vastly less significance to the members 

 of the genus Solanuin, in which, indeed, it can only 

 be looked on as exceptional. The object of the 

 cultivator should be directed, not so much to the 

 conditions under which the members of the genus 

 Solanum naturally grow, as to the 

 peculiar conditions which have 

 caused one particular species in an 

 especial manner to develop tubers. 

 It must never be forgotten by the 

 thoughtful cultivator that particular 

 plants— wild plants — do not as a rule- 

 gTOw under the conditions which are 

 the most favourable to them, but 

 where they can hold their own in 

 face of the severe competition to 

 which they are exposed, and in spite- 

 of the many adverse conditions by 

 which they are handicapped. This 

 truth was long ago insisted on by 

 Dean Herbert, one of the foremost 

 physiologists and cultivators of this 

 coimtry, and it is a truth that 

 should be full of encouragement to- 

 the gardener, who often has it in his 

 power to remove any given plant 

 from the struggle Tsith competitors, 

 and not only to banish adverse con- 

 ditions, but to place the plant under 

 the most favourable circumstances 

 possible for its development. This- 

 is what has been done partially for 

 the Potato ; partially, because only 

 with reference to the production of 

 tubers, the tendency to the formation 

 of which has been by the art of man, and the- 

 plasticity of the plant, enormously increased. But 

 this development of one organ has been obtained at 

 the expense of some compensatory drawbacks, fore- 

 most among which is an enhanced tendency to 

 become diseased, or at any rate a diminished power 

 of resistance when attacked. 



The tuber of the Potato is nothing but the 

 thickened end of an under-ground branch. As it is 

 destined to remain under-ground, no true leaves are 

 formed, and therefore any food it may get is con- 

 veyed to it, not formed by or in it. That it is a 

 branch is shown by its origin from the stem or 

 haulm, and this is further shown by the presence of 

 the eyes or buds. The Fir-cone Potato, a variety 

 occasionally met with, resembles a long Fir-cone 



Section of Corm of 

 Crocus. 



