41 



LECTURE XXVI. 



of a new period of vegetation, without nutrition occurring at the same time. This 

 fact may perhaps be demonstrated still more plainly by simply allowing seeds, 

 tubers, and bulbs, thoroughly saturated with water, to lie or hang in moist air, 

 where the germinal shoots and roots make their appearance, and grow up to a 

 certain point, without taking up nutritive substances from without, but only with 

 the aid of the respired oxygen and the heat supplied by the environment. 



Growth and nutrition thus by no means coincide : but it is self-evident that 

 growth can only occur if constructive materials — plant-substance capable of 

 organisation — are already present. It follows from this fact, that we can never 

 infer simply from the occurrence of growth a simultaneous occurrence of nutri- 

 tion ; and in the same way it is by no means to be concluded from the fact that 

 nutrition is proceeding, that the organs of the plant are at the same time growing. 

 This appears most evidently in the organs of assimilation — the green leaves them- 

 selves — the nutritive activity of which only predominates when they are themselves 

 completely developed. If, now, the growth of a single leaf, a shoot, a flower, or of 

 any other organ is observed from its origin up to the moment when it is completely 

 developed — the developmental history, as it is usually termed — two points are 

 noticed, viz. on the one hand the volume becomes larger and larger, until it finally 

 attains a definitive size and increases no more ; and, on the other hand, the form 

 of the organ, at first sketched out only in its coarsest outlines, so to speak, becomes 

 further developed and more finely elaborated during the increase in volume, until 

 finally its definitive form is perfected. 



Growth is thus an increase in volume, closely combined with a change in form. 



It may perhaps contribute to the elucidation of the idea if we compare the 

 'growth of a plant thus defined with that of a crystal, on the one hand, and with 

 that of an animal on the other. 



In the case of the crystal, two points are also to be distinguished in the 

 growth : the increase in volume, and the formative forces. From the material in 

 a dissolved, or fused, or even gaseous state, minute and scarcely visible crystals 

 are formed, which then increase in size. But in their growth the geometrical 

 form given from the first remains unaltered, and the increase in volume is not, as 

 in the case of plants and animals, a specifically limited one. Under favourable 

 circumstances a crystal may go on growing further and further; it is never, like 

 an organ, 'fully grown.' Moreover, the mode of growth is essentially diff'erent; 

 for in the first place a crystal takes its origin from amorphous fluid matter, and 

 then grows by means of the deposition of new, minute, invisible particles on its 

 surfaces. A vegetable body, on the other hand, never originates directly from 

 a fluid, but always as part of an existing and already formed plant-organ ; or, if 

 one will so express it, all crystals arise by ' spontaneous generation,' which is never 

 the case with organisms. Crystallisable substances may completely give up their 

 form on solution, fusion, or evaporation, and assume it again under suitable external 

 conditions : organisms, on the other hand, exhibit an uninterrupted continuity of their 

 existence. The substance of a plant or of an animal dissolved and in an amorphous 

 condition never resumes the organised form : this only proceeds from an organised 

 substance already present. 



With reference to the increase in volume, also, a fundamental difierence exists 



