2. Die Hybriden derselben müssen während der Blüthezeit vor der Einwirkung jedes fremdartigen Pollens geschützt sein oder leicht geschützt werden können.
2. Their hybrids must be protected, or be easily protectable, from the influence of all alien pollen during the flowering period.
Their = derselben The reference of the demonstrative pronoun in the German original is ambiguous; it can refer to Merkmale or Versuchspflanzen in the previous sentence. Bateson goes for the latter option ("of such plants"), we follow Sherwood.
alien = fremdartigen Literally translated, this compound adjective means “of alien kind”, so it has a taxonomic dimension as well in the sense of “belonging to a different kind” or “species”.