IRRITABILITY. 589 



more perfect differentiation of the tissues, and the develop- 

 ment of the leaves proceeds. This leads us to consider the 

 relation of the growth of leaves to light. We have learned 

 (p. 380), that in cases in which excessive elongation of the 

 stem takes place in darkness, the leaves are imperfectly 

 developed, and we found ourselves at a loss to explain, in any 

 direct way, the fact of the rudimentary condition of the leaves. 

 The explanation is that, under these circumstances, the deve- 

 lopment of the leaves would be of no advantage to the plant : 

 it concentrates, as it were, all its efforts to the elongation of 

 the stem to the end that light of sufficient intensity may 

 ultimately be reached. We found also, in the case of plants 

 in which, under similar circumstances, the stem does not 

 become excessively elongated, the Onion for instance, the 

 leaves become so. The significance of this is the same as 

 that of the excessive elongation of the stem : the leaves 

 elongate in search of light. 



Another instance of precisely the same nature is afforded 

 by climbing plants, whatever be the means by which their 

 climbing is effected. The stems of these plants are not suffi- 

 ciently rigid to enable them to support the leaves in the most 

 favourable position as regards exposure to light. Consequently 

 they attach themselves to supports and thus attain the required 

 end. In some cases the attachment is of a mechanical nature, 

 as in hook-climbers ; in others existing parts of the plant 

 possess special irritability, as in leaf-climbers and stem- 

 climbers, or altogether new and irritable organs are developed, 

 as in tendril-climbers. 



It may perhaps be objected that it is not clear, from this 

 po'int of view, why, as we have seen (p. 385), the shoot-organs 

 of plants destitute of chlorophyll should grow more rapidly in 

 darkness than in light. In reply to this objection it has to be 

 pointed out that these plants have undoubtedly sprung from 

 ancestors which contained chorophyll : for instance, the Fungi 

 have been derived from the Algae. Now derivative forms 

 retain, in a greater or less degree, the more general properties 

 of their ancestors, even though these properties may be 

 of comparatively little advantage to them, and the case before 



