176 COOK 



the new growth appears, whether of flowers or branches or of 

 true functional roots. A cluster of spine-like leaves or branches 

 in this position would need to represent a rudimentary branch 

 or series of branches. The arrangement of these would be 

 spiral or concentric, which is evidently not true of the spines.^ 



APOSPORY AND ALTERNATION OF GENERATIONS. 



In comparison with the current application to the higher 

 plants of the doctrine of alternation of generations, the present 

 suggestion may also claim merits of simplicity and directness. 

 It is no longer necessary to imagine that two generations orig- 

 inally alike have become specialized, nor that one generation 

 is borne parasitically on the other except in the manner in which 

 the aposporous fern prothallus is borne. 



The evolutionary history of the angiosperms can be thought 

 of as a definite progressive sequence, without the need of sup- 

 posing that one " generation " has deteriorated while another 

 has expanded. The macrospores and the vegetative prothalli, 

 having been completely eliminated from the life-historj'- of the 

 primitive angiosperms, did not have to be gradually reduced 

 from a functional to a rudimentary condition. 



The so-called endosperm of the true seed plants need not be 

 supposed any longer to correspond to the prothallial endosperm 



' Professor Edward L. Greene has pointed out to me that the spine-clusters of 

 Mamtnillaria are similar and perhaps analogous with those of certain of the 

 species of Mesembryanthemum, which represent marginal spines of the reduced 

 blade of the leaf, while the conical prominence upon which the cluster or com- 

 pound spine is seated may represent the petiole or base of the leaf. The flowers 

 and fruits of Mamtnillaria do not emerge, as in Opuntia, from the same point 

 as the spines, but from the axils of the fleshy prominences which bear the spines. 

 But Professor Greene considers the assignment of Matnmillaria to the same 

 family with Oputitia an artificial arrangement, so that an analogy between 

 Mammillaria and Mesembryanthemum would not necessarily interfere with the 

 recognition of the capped Opuntia spines as roots. That the spines of Opuntia 

 and Cereus are not to be interpreted as equivalents of marginal projections of a 

 reduced leaf-blade, seems to be proven by a fact observed by Dr. J. N. Rose, that 

 in these genera new spines may be added one after another, even in very old 

 clusters. Nor is it likely that new leaves would be pushed out in this manner, 

 which is much more appropriate for branches or for roots. That the thorns of 

 some of the cacti secrete nectar, as shown by Ganong, does not exclude the pos- 

 sibility that they are roots. Exposed root-tips often produce secretions, and my 

 son Robert informs me that the secretion of the aerial roots of the corn plant 

 has a sweet taste. 



