474 



HETEROMORPHISM AND ALTERNATION OF GENERATIONS. 



The impulse to the production of flowering-shoots cannot entirely depend 

 on the prevailing climatic conditions of t'le year in which the flowering takes 

 place. For in the autumn of the previous year the bud is already laid down, 

 and one can tell by dissecting it whether it will form a flowering or a foliage- 

 shoot. In associating climatic conditions with flower-production, it is the summer 

 of the year previous to flowering which must be taken into account. This is 

 well illustrated by the seasons of the years 1893 and 1894. The summer of 1893 

 was, as is well known, remarkable for its warmth and long-continued sunshine. 

 This was followed in 1894 (to take an example to hand) by the flowering of many 

 plants in Kew Gardens which are hardly ever known to flower there in the open, 



under ordinaiy circumstances. Of these it will 

 be sufficient to mention two Gymnosperms, 

 Ephedra and the Maidenhair tree (Gingko biloba). 

 It is easy to observe the fact that in a big 

 tree, of which one side is in the full sun whilst 

 the other is shaded, the shady side produces 

 foliage-shoots for the most part, whilst the sunny 

 side blossoms freely. Nor can one resist the 

 conclusion that it is the sunshine which stimu- 

 lates the flowering. The same thing is shown 

 by plants, which, growing in dense forest shade, 

 remain without flowers from year to year: but 

 as soon as the trees about them are felled, and 

 the light gains entrance, form flower-buds, and 

 ultimately blossoms and fruits. The advantages 

 accruing to the plant by this change in its sur- 

 roundings have already been indicated on pp. 394 

 and 459; but what immediate influence the sun- 

 light has on the building capacity of the plant, 

 and how it is that the tissue which, in the shade forms a foliage-bud should in the 

 sunshine form a flowering shoot, must for the present remain unanswered. 



Fig. ZiS.—Iihipidoptefispeltata showing sterile 

 fronds to the left, and fertile ones to the 

 right. 



And now, as regards Alternation of Generations. The relations between the 

 sexual and asexual generations are very various in different portions of the vege- 

 table kingdom. In some groups of plants the two generations are obvious and 

 distinct, in others it is very difficult to draw the line between them. In the Ferns, 

 Horsetails, and Vascular Cryptogams generally, the two generations are quite 

 distinct and easily recognizable. In the Ferns the generation which bears the 

 sexual organs ( = sexual generation or oophyte) takes the form of a small, expanded 

 plate of cells, from the under side of which delicate hair-like rhizoids are developed 

 which penetrate the soil (see flg. 189 '^ p. 11, and fig. 346'). This plate-like 

 structure is usually known as the prothalUum; it is either heart-shaped or ribbon- 

 like and lobed, attaining a length of from '5 to 1 centimetre. The sexual organs 



