3«° 



LECTURE XIX. 



separating layer, which melts in the rays of the morning sun, whereupon the leaves 

 suddenly fall in numbers from the trees. 



There can be no doubt that the contents of the assimilating parenchyma 

 of the leaves before the fall are taken up during the changes described into 

 the persistent parts of the plants— into the cortex or young wood of the branches. 

 I was able by micro-chemical methods to follow distinctly the travelling of the 

 materials, especially of the starch, out through the tissues of the petiole into the 

 shoot-axes ; and moreover the ash-analyses of assimilating green leaves, compared 

 with those of fallen ones, show that the most valuable mineral constituents of the 

 leaves, specially the potash and phosphoric acid, also pass out through the leaf- 

 stalks simultaneously with the organic substances, and back into the parts of the 

 shoot which survive the winter, evidently to serve, like these, as nutritive matters 

 for the newly sprouting shoots in the next period of vegetation. 



While this process repeats itself each Autumn in perennial plants, it only 

 takes place once in the annual summer plants, and, generally, in those plants 

 which only fruit once. In our cereals, for example, and other cultivated plants, 

 all the still useful materials which were contained in the leaves and shoot-axes 

 are collected finally, when the fruit is maturing, in the ripening seeds, either in 

 the endosperm or in the large cotyledons of the embryo, to be employed later, 

 on the germination of the latter, as materials for the construction of the growing 

 parts of the seedling. Hence the vegetative organs of such plants after the ripening 

 of the fruit (generally named straw, etc.) contain only exceedingly small quantities of 

 material capable of being employed for further growth : they consist of emptied 

 net-works of cell-walls, with slight remnants of other matters. Of ash constituents, 

 the whole of the silica, as well as the greater part of the lime in the form of 

 calcium oxalate, remains behind, as a rule, in the emptied organs of assimilation. 



The processes are somewhat different in the chlorophyll-corpuscles of those 

 organs which are not to be regarded 'like the leaves as assimilating organs in the 

 proper sense, and the chlorophyll-corpuscles of which are probably only green 

 starch-forming corpuscles in the sense of Schimper. I found, for example, that the 

 chlorophyll-corpuscles in the antheridia of Nitella and Chara, as well as those of 

 some Mosses, which enclose starch in the unripe state, assume a red colour when 

 the antherozoids are mature, but maintain their form, and the enclosed starch does 

 not disappear : profound chemical aherations of the plastic substances do not, how- 

 ever, take place here. Much more thorough is the destruction of the corpuscles, 

 at first green, in the pericarp of those berry-like fruits which appear red or deep 

 yellow in the ripe state, e. g. Lycium and various species of Solanum, The chlo- 

 rophyll-corpuscles of these pericarps, as they turn yellow and red, change their 

 form also : they become angular, two and three pointed, and finally break up into 

 small granules. In conclusion, the remark may be made here that the bearers 

 of the yellow colouring matter to which many floral leaves owe their yellow 

 colour (e. g. the corolla of Cucurbiia), resemble chlorophyll-corpuscles, or better^ 

 Schimper's starch-forming corpuscles. 



When organs of assiniilation become dormant at the conclusion of a period 

 of vegetation, to renew their activity in the following year, alterations take place 

 in the contained chlorophyll also : under certain circumstances these however 



